Offset function with a dynamic Height range

Hello

In excel 365

I am trying to create a dashboard of a rolling 6 month sales average.

I am using this function =AVERAGE(OFFSET($C$1,COUNTA(Sales),0,-6,1)) where sales is a named range with sales data.

I would like the height in offset formula currently set at -6 to be dynamic to pull in an average of 6 months or less in situations where 6 months of the fiscal year has not elapsed.

Exe return 6 month avr or 5 months avr in currently in May or 4 month avr if currently in APR or 3 month avr if currently is March