Debt Service Test - Payment Known, how do I find Principal Balance?

I am trying to find out how much principal I would be required to pay down to meet a DSCR test. So, if I know my NOI is $1M and my required coverage is 1.10x, then I have a total payment (P&I) of $909,090. How can I solve for the principal balance that would give me a total payment of $909k during that period?

My test is based on 7% P&I and 360 months. I know I can use Solver but what if my NOI changes or if I want to run sensitivities. Is there a formula?

I solved my own problem. It can be done by first finding the debt constant, then the desired debt service, then divided the desired debt service by the constant. That's your paydown.