I am trying to enter a formula to calculate how many periods it will take to payback. The formula I have works well unless someone enters additional capex at a later date - then it all goes to pot.

The original calculation is on sheet 2 - but I'm playing around with sheet 1 to try and get a formula that works.

If anyone could help, or has any suggestions, they would be very much appreciated.

https://drive.google.com/file/d/1lambPuZxLW_WneONt3EO4yPgz38p-Sfz/view?usp=sharing