context of 1 Cor. 16:2 is a collection for the poor, heavily persecuted believers in Jerusalem. Paul wasn't even going to pick up donations and bring them there; he instructed that someone from the group in Corinth be chosen themselves to be entrusted with it. see verse 3.
notice how he doesn't want there to be any collecting of money going on while he's present? why not? wouldn't people feel more compelled to give during all the excitement while Paul was there?
It is an example of how they gave to support a cause.
Another cause is the support of the local church. Why not use the same principle. "How much should I give each week?" How about a proportion as God has given you? "When should I give it?" How about each time you meet give in proportion to how God has prospered you since the last time you met.
It is not complicated. It is not a law. It is a method. One that was used for support the poor in the church in Jerusalem.
The idea of it being a proportion as God has prospered you is a good method and it comes from the bible and is one example that we can point to and say "Look what they did there. that sounds like a good way to do it." And just doing it. The main thing is that you support the local church because you love the church and want to keep it functioning.
Now you could also do it to send a Missionary out. Or to support a Pastor who is going to plant a church. Or to help some saints in a church in Africa get a water treatment plant or a well dug so they don't have to drink parasites. Give a proportion each week until the collection is ready and send it.
The proportion as God has prospered you is a biblical method of how much to give and how to do it regularly. "What proportion? How much?" It is in accordance to how God has prospered you. If you chose a percentage like 10% that might be a good method for you as you can generally make it on the rest. But what if you were extremely poor. 10% might be more than you could afford. Give what you think you can. What if you were abundantly blessed with unexpected income, you might want to give more than 10% and you were blessed with so much that even 50% would leave you with much abundance for yourself and you are excited about helping the church with a large sum. This is in proportion as God has prospered you.
I know someone who received an unexpected $8000.00. They received it at the exact time that their church was experiencing a financial crunch. There were no appeals given and no one in the congregation new about it, they heard about it because they were part of the weekly staff meetings where such things were discussed. They were talking in the staff meeting about what they could cut in order to meet the basic bills. When this unexpected income came to them they believed that it was for the purpose of helping the church meet that need. They believe that this is God's method. Unexpected prosperity will come to individuals in the church so that if they are all being faithful to give a proportion of that surprise income to the church they will meet a need that just so happen to be faced by the church at that time and they don't even know about it. They gave $5000 of the $8000 to the church and used the rest to pay taxes on the $8000 and to take car of a debt they owed. This to me is a good real life example of how individuals who understand the principle of 1 Cor 16:1 are used by God to fund the church.
The Lord may call a pastor to start an outreach to the community that will cost money but will be effective in winning the lost. They may know that they have heard from God but don't know where the money will come from. Then members in their congregation will get surprise income from various sources. If those members are praying and if they are spiritual mature and understand this principle will give in proportion as God has prospered them and it will meet the need of that outreach ministry goals.
This is God's method. He will bless individuals with finances because he wants them to give a proportion to the church. THAT IS WHY THEY GOT THE SURPRISE INCOME. Only a small percentage of these people get it and give anything. They go about telling everyone how God blessed them with unexpected money but they don't give a proportion of it to the church. They may only give 10% out of a legalistic rule, and this is up to them but if they were sensitive to the Lord they might have given more. It is very rare to find people who understand that financial prosperity is coming to them to give to the work of evangelism. (The eye of the needle)
If they meditate on 1 Cor 16:1 a light should go off and they should have that "AH HA moment" God gave this to me so that I would give a proportion because there is a need in the church and this is His way of meeting it. This is God's method. Each member giving a proportion as God prospered them during the week.
After this letter was read many of the the individual Corinthian saints probably experienced financial gains that week and thought to themselves... "I know why I got such an unexpected boost of income! It was so that I could give a proportion of it to meet that need that Paul told us about." This is how God has ordained that his Church should function.