I got you.
So here is how the math works.
Imagine S being a set of bonuses that are noted as pairs of bet and result. U is a subset of those elements in S which have a result lower than €0, as to mark them as not opened yet. O is a subset of S which have a result of €0 or more. Then we also have the starting balance and the current balance.
With those balances we can calculate the amount to break even. And with that amount and the amount of unoponed slots we can calculate the break even amount every slot has to contribute to break even in total.
With this break even amount per slot and the betsize of each slot we can calculate the break even x every slot has to contribute with respect to their betsize to break even. It's that easy.
And here is some formulas I came up with. In the last line I pulled the breakeven amount per slot out of the sum formula and substituted break even per slot with the total break even divided by the amount of unopened slots because that formula looks much better than the longer ones. It also eliminates the need to calculate the break even per slot. (But as always you sometimes have to go a longer way and come up with long formula jsut to see in the end that you can crunch them down even further)
Example with 3 slots and the first slot already opened:
Hope this makes it now even more clear how it is calculated.
Have a good day!