Thanks for your formula.
Question 3 refers to the first positive cumulative cash flow.
Adding 1 to this figure to reflect the first positive month increases a
pay-back period because the pay-back point is somewhere during the first
positive month, not at the end of the month.
"Dave O" wrote:

Questions 1 and 2 are related: if you calculate payback by counting
months where cumulative cash flow is less than zero, you can use this
formula to count negative values in a range:
=COUNTIF(A1:L1,"<0")
.... where months 1 - 12 are in A1:L1. You may need to extend this
range if your timeline requires it. Also, add 1 to this figure to
reflect the first positive month.

Does question 3 refer to first positive month number, or the first
positive cumulative cash flow?