You could determine the point during the month if you have weekly or
daily data to use in the calculation. Or if, for example, the first
cash positive month is month 8, how about
(cum. cash flow month 8) / (revenue month 8)
That would return a fraction of month 8, but it assumes that daily or
weekly revenue streams are even.

Just a heads up: this "count the negative months" logic is correct, up
to a point, but it may mislead your audience. Since cumulative cash
flow can swing from negative to postive and back to negative,
particularly if there are capital expenditures in the outmonths, anyone
who reads your report may assume that there are an unbroken number of
cash negative months followed by an unbroken number of cash positive
months, which is not necessarily the case. This is difficult for
non-finance types to understand, and you may need to explain that to
your audience.