I need to calculate the payment required to return x% IRR to a schedule of
investors. We have an irregular series of cash flows, so it's easy to use
XIRR and find the final payment value using Solver.

How do I calculate the final payment, referencing my x% return, for all
investors without running solver repeatedly?

I wrote a formula that does this, but it's sheer brute force. Is there a
trick to handle this more easily in the future?