You commented only my last wondering ...
... basic sum-function needs those Your tables ... okay?
>>> Why You have those AH6:AJ11 -values?
My sample ...
Check Sheet2
Below [ DoIt ] ... have those manually given 'months' (( Bold Black ))
Below tot ... have those manually given values as 100, 50, 80, 150,120 (( Bold Black ))
... there could be more of those
More right side...
You could modify those A, B & C as You would like to (( Black ))
... and You could add there of those too.
Below of those A, B & C...
there can be ONLY number of USED rows values ((
Red )) (each rows sum gotta be 100%)
The most right side is Your original %-table.
Those columns can delete.
Usage
After modified/verified 'months', tot and right side data
... press [ DoIt ]-button
Note: I have NOT checked everything