I like this technique...
Create two new worksheets--one to the far right and one to the far left.
Call them Start and End
Then using a sheet (Summary) that is outside this "sandwich" of worksheets:
=Sum(start:end!A1)
Then you can drag sheets in and out of that sandwich to play what if games.
I'd put a couple of notes on each of these sheets:
"don't delete this sheet!"
And protect the worksheet so that people don't use it for real data.
So if you put 18 tabs outside that sandwich of Start:End, those values won't be
included in the summary sheet.
(And just drag them in when you need to.)