Multiplying by First Cell Over Zero

Wade

I could tell you how to determine the first non-zero figure in a row but I don't think that's needed here since I've taken a different approach in the attached revised sheet.

I've hidden your columns AA:CX and made column CY the same as B (so the 1790 figure, 0 or otherwise) e.g. cell CY1 has formula:

=B1

and the same down the column (6 or however many rows).

Given you want to increase any calculated population by any positive percentage  increase in a decade (or not increase it is the raw data value fell), in CZ2, the revised formula used nexted IFs and is:

=IF(B2<=0,C2,IF(C2>B2,CY2*C2/B2,CY2))

so it increases the numerical value figure in the column to the left by any positive percentage increase in population (in the raw data) or gives the raw data if it's a(/the) first non-zero value.. If the 1800 figure is less than the 1790 one, it doesn't change the value (it stays as per the previous 1790 column). This is then copied down and across, over the decades.

You'll see that the formula produces values in green cells CY3:DV3 which match your B9:Y9 sample values (but adds the original non-zero population)

You'll see that I've added an additional formatting rule so any decade-on-decade value drops appear yellow in the raw data and in the calculated data show that the value was held at the previous figure.

Hope this is what you need. Kindly mark this answer as Selected if so.