I have a spreadsheet with various values in three columns. Each column is a
different month of the yr. The values in some of the cells are used to
calculate ratios which appear in let's say row 10.

C D E
F
Row 1 2 5 10
Row2 8 9 3

Row 10 =C1/C2*365 =D1/D2*365 =E1/E2*365 in F10 =AVERAGE(C10:E10)
and in column G through all the other months it is currently using that
same average.

If there are no values in the column needed to do the calculation then e.g F
here, then the cell should average the previous months and use that average
for future months. If the cells/ columns for that month do have values then
it should calculate using the relevant formula and then adjust the next
months cell with the new average e.g. G in this case would then average
c10:f10 and not to e10.

Should I be using the averageif function and how do I use it in this case?
Thanks!