What is the proper allocation of the tithe? Do I deduct it from my net or gross salary?
The Bible has much to say about Christian giving, but the New Testament nowhere mentions tithing as a Christian obligation. Clearly, it is in the interest of many organizations to have a large and regular stream of contributions from their adherents, but tithing per se is a requirement levied upon the ancient nation state of Israel, not upon contemporary followers of Jesus Christ (no matter what their nationality).
Please see the following links for the details on this:
Tithing and the Book of Life
Is tithing required for salvation?
The 10/80/10 model?
Melchizedek and tithing
We know that God does not want His people to give reluctantly and out of compulsion, but wants us to be happy about giving in support of the Church of Jesus Christ (2Cor.9:7), just as the Macedonians did, who contributed to the impoverished Jerusalem church with great joy and beyond their means (2Cor.8:1-7). We know that our giving is evaluated by God by this standard rather than on any percentage basis, and that gifts which truly cost us dear are more blessed than large amounts from those whose means are excessive (Mk.12:41-44).
Tithing a tenth of one's crops was a good system for supporting a national religious establishment for a country in an entirely agricultural economy (and even here it is not so simple as a once a year 10% system: see Lev.27:27-30; Num.18:21-29; Deut.12:6-18; 14:22-29; 26:12). But tithing is not really workable in the church, in my view, and for that reason is never commanded or recommended or referenced as a church practice in the New Testament epistles. The Pharisees were very proud of their tithing, even going so far as to tithe the produce of their spice gardens, but Jesus condemns them for their hypocrisy on this point (Matt.23:23; cf. Lk.18:12), noting elsewhere that they deliberately tied up their valuable property by "dedicating it to God" so that they would not have to use it to help anyone, not even their parents (Matt.15:5). This really is at the heart of the issue of tithing. We Christians are called on to help each other. This might be easy sometimes, and sometimes it might be hard. Nowhere in the New Testament are we given the impression that we can or should stop when we hit 10% (whether net or gross). Some have received the gift of giving and the resources to go with it - they are commanded to give generously (Rom.12:8; cf. 1Tim.6:3-10; Jas.5:1-6). Some Christians are even wealthy - they are commanded not to trust in their wealth but to share generously (1Tim.6:17-20). And our Lord commended to us all the example of the "shrewd manager" to show us that worldly wealth is only really of any use if it is put to use for the Kingdom of God (Lk.16:1-13; cf. Prov.17:16). On the other hand, there are many Christians who are poor, and many who though they might not consider themselves poor have very little "disposable income". If these are rich in good deeds and in the work of the Church of Jesus Christ, do we really suppose that they will come under divine displeasure for being unable to give 10% (net or gross)?
The phrasing of your question really makes this whole issue fairly clear. Were I a rich man, I am sure that I could so arrange my estate that my "income", however defined, would be relatively small, tying up my wealth in long term investments and tax shelters, just like the Pharisees found a "solution" by having their estates classified as "Qorban". But if I had an abundance of the means of life and saw my brother in desperate need, would I really be doing right to harden my heart against him and ignore his plight, even if I had already contributed 10% of my reduced income to some charity (Jas.2:14-17; 1Jn.3:17)?
As believers in Jesus Christ, we have been freed from the Law of Moses and its various and sundry regulations like tithing (Rom.6:14-15; 7:4-6; 10:4). But we have not been freed from the law of love, for love is the fulfillment of the Law (Rom.13:8-10). We Christians have freedom and liberty that those under the Law did not have, but we are responsible to use our freedom for good in every way (Gal.5:13; 1Pet.2:16). Therefore if we are truly walking in the Spirit, and responding to the love of God, we will not be wanting in opportunities to do good for the Body of Christ, and, God helping us, we will not prove wanting in seizing them - even if it means the sacrifice of much more than 10% of our time, our effort, our energy, our persons, and even our material resources. We have each been given a spiritual gift and a way to put it into practice once we have attained a certain level of spiritual maturity. Let us therefore resolve to use these lives, these bodies, these opportunities, and these resources given us by God in the service of His Son, our Lord and Savior Jesus Christ - for to this we have been called (Eph.2:10).
In Him who made Himself poor for us that we might receive all things, our Lord and Savior Jesus Christ.
I have a question on your translation of Jesus' words in John 8:58 as "I am". The Watchtower organization points out that other "I am"s in the bible are translated as "I have been" (and argue on this basis that we should translate John 8:58 likewise). Such as when Jesus told Philip "am I with you so long, Philip, that you ask to see the Father?" And it's usually translated as "Have I been with you so long....". But I think that the context is different, the meaning of the two versions is the same, and putting it into good, vernacular present perfect tense doesn't change the meaning one iota. But translating John 8:58 as "I have been" does.
I think your comments on John 8:58 and the Philip parallel are right on the money. People who don't deal with these things for a living often fail to grasp that translation involves understanding the passage and then rendering it into the target language in a way that conveys the meaning, sense, and tone as well as possible - but that does not grant a reverse carte-blanche to use said translation in any and all situations. The verb gignomai is one of the most common verbs occurring in the N.T.; it can mean and sometime does mean "to be born"; that does not mean we have the right to translate it that way every time it occurs (a circumstance which would yield some very bizarre renderings, like: "and a number of men were born, about 5,000", Acts 4:4). These discussions reveal very quickly the people who are genuinely seeking truth from the Word of God, and those who are only looking for ammunition to defend their preconceived positions. Jesus is clearly claiming to be divine in John 8:58, and no amount of reverse translation based upon English equivalents can deflect the straightforward meaning of that passage.
In the God of all truth.