I'm not sure I quite understand.
I think what you are asking to do is
First Lookup the months as one lookup then
Next Lookup the number of new accounts
And come up with the KPI rating from those two individual lookup results?
Also, I see 2 columns of New Account values - which are we supposed to be looking at (B or C) to find the value in the lookup of new accounts.
By the way - check your post for accuracy: you say Jane has been in her role for 10 months and has 40 (???) new accounts? Should that be 20 or 44?
Personally, at this point I'm thinking that in the long run the easiest thing to do is create that huge table you're dreading on setting up and use either a SUMIFS() or older SUMPRODUCT() formula to pull data from it.
It might help if you can show us the 'scale' chart you mention and explain how that works. Best might be if you can upload a sample file to a file sharing site or to your OneDrive and share it and provide a link to it in a posting here.