Formula to find number that yields zero in another cell

Hello!

I'm working on a calculation that requires finding a number in one of the cells that will yield zero on a check cell. The latter compares the result of the calculation to a value brought from a different formula.

There are two cells in the formula that are calculated with percentages. I already enabled iterative calculations to allow circular references.

See enclosed worksheet. I filled in cell E8 manually, but that is the value I need the formula to calculate automatically so that cell E28 yields zero.

I cannot find the formula that will allow this calculation

Thanks for your help!