cellA - cellB if result 0 then result in locked cellC

12 sheets named JAN-DEC, rows R15-R38, column "J15" = previous sheet "R15" - "T15"(except for sheet 1 - no previous)
if "J15" > 0 then lock cell and show result, if "J15" = 0 unlock cell for new entry and show 0
Columns "I" & "K" are empty and used for spacing - if needed, I currently have code in sheet Activate & Click - no problems in adding
Workbook locked pass = 123, sheet protected = 123

Unsure of if needing a function in cell + a sub/function, not enough knowledge on where to start - some coding background.
thanks in advance...

I have solved about half of this, was looking for an efficient way to do this, will mark as solved.