I have a sheet with strength (no of pers) in column b

i have no of separations in column c

in a1 i have the following formula - =SUM(C202:C213)/((((B201+B213)/2)+B202+B203+B204+B205+B206+B207+B208+B209+B210+B211+B212)/12)

c202:c213 12 months of seps data/2

b201+b213 is the first and last month over 13 months

+b202 through b212 is the remaining 11 months strength

This formula gives me a rolling separation percentage.

I need vba code to automatically select the last 13 months on data in column b and last 12 months data in column c and apply my current formula, this will save me to change the range each month , more importantly i will leave no room for error.

any ideas would be appreciated.

Cheers