You are right that a simple excell chart is plenty to make any complicated mathematical formula into a simple: add a number, take out result.
But why would you want to make the idea somewhat complicated, so you need a spreadsheet, when you can keep it simple enough for every participant to calculate it him/herself?
Many folks may like to know how much money they will get in the next round for planning ahead, without having to ask a mod / using spreadsheet each time.
I like the idea and it looks like you've put quite some thought into it. But sadly not everyone is with mathematical skill gifted. So IMO going down to the basics is the best option, especially since the resault is exactly the same.
Add a number, take out the result seems pretty simple to me. And that would also take out the human factor of it (i.e. miscalculations).
I guess having to look in a spreadsheet to know how much money you'll get next round is too little drawback to be worth the human mistakes it prevent.
