I know tithing is not applicable under the New Covenant, but I was wondering what your view is on what exactly is gain or profit if one only receives wages.
Is it only the amount by which they receive a raise on their paycheck each year? If so one could argue that it is (% Raise - % Inflation) or ($ Raise - $ increase in your cost of living). In today's world, raises are really not a gain if you consider increases in the cost of living. Anyone that lives in the real world knows that government figures on inflation are far from the real life cost of living increases.
Is it the amount of your income left over after you pay for basic needs - food, clothing, shelter, medical? One can't survive where I live without a car, so I consider that a basic need since I would end up in jail trying to ride a horse to work. If this is the case and someone wanted to estimate a gain for the year, I would think they would probably have to use an average figure to base it on and not actual money spent since the variance is too great to be fair.
Anyway it would be nice to have an idea for this so if one was discussing the tithe with someone, they could at least point out that not only is tithing not Christian, but they are being duped into actually giving far more than a tithe and they don't even realize it. I find it interesting that some denominations not only call for a tithe, they will then take up special collections on top of that for Bible study materials and maintenance. What a great scam!
"On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come" (I Corinthians 16:2).
"For if there is first a willing mind, it is accepted according to what one has, and not according to what he does not have" (II Corinthians 8:12).
"But this I say: He who sows sparingly will also reap sparingly, and he who sows bountifully will also reap bountifully. So let each one give as he purposes in his heart, not grudgingly or of necessity; for God loves a cheerful giver" (II Corinthians 9:6-7).
There is no fix amount or percentage given. It is what each individual decides and the amount is to be based on the prosperity of the individual. If the economic situation eats away at a person's income, one expects him to give less. If he has been well blessed by the Lord, one expect him to be giving more. God did not give greater details and I'm not going to go beyond what He said.
But I will point out that in your equations, notice that you are figuring what to give God from the leftovers. God told the Israelites to give of their firstfruits. In other words, they were to give to God first and then live on the rest. I use that in my own decisions. My giving to God has always been the first budget item. "Now may He who supplies seed to the sower, and bread for food, supply and multiply the seed you have sown and increase the fruits of your righteousness, while you are enriched in everything for all liberality, which causes thanksgiving through us to God" (II Corinthians 9:10-11).
Sometimes we focus on our personal situation and forget that the church and those supported through the church face the same problems. When the economy goes bad, it means the funds raised don't go as far as they used to, so less is purchased. As an example, I work full-time preaching, but when the economy went bad, I had to take up a part-time job to make ends meet. Not that I'm complaining. I'm fortunate to have a part-time job that I can fit around my preaching. But sometimes people don't realize that less given to a local church impacts what it can do and who the church is able to support.