Trying to sum values along a table's hypotenuse

Hello - long time lurker!

I'm trying to add up values along essentially a hypotenuse from a pivot table (see screenshot and attached).

I have manually entered a basic formula similar to what is in attached spreadsheet in the past but my table on this new project is too big.

Any ideas on a lightweight formula that I can use in column B and drag down? Was thinking about some sort of offset but couldn't make it work.

Screen Shot 2020-07-17 at 1.52.02 PM.png

Solved w/ a better search of the forum!

Thanks all!

Thanks Geoff - I'll give your approach a go today as well.

I found my other solution here on excelforum under this thread: Find SUM of diagonal entries in a range of values.

Here is the array formula that I'm using which also requires helper columns: {=SUM(IF((ROW(INDIRECT(D13))-ROW(INDIRECT(A14&":"&D14)))=COLUMN(INDIRECT(A14&":"&D14))-COLUMN($AI$11),INDIRECT(A14&":"&D14)))}