Ok, I figured it out after a little tinkering and narrowing the problem!
My concern was allocated but unspent funds. Spent funds would already be on the credit card or out of our primary checking account, so I didn’t actually need to figure those in the calculation because they’re already in Savings Budget. That was part of what was tripping me up.
SUMIF became my friend here. I used SUMIF to add up any categories where the available amount was positive and ignore ones that were negative with the formula =SUMIF(Range,“>0”,Range).
I referenced the balances of my primary checking and primary savings. Then I referenced my credit card balance, created a cell for an emergency fund reservation. I then added together the credit card balance, emergency fund, and total of the reserved but unspent categories, and subtracted those three things from the total of checking and savings, and got the number I’ve been looking for!
(Please have mercy for all the massively overspent categories. This is precisely why I needed to figure this number out!)
This lets us know that if we want to go over in a category for the month because we think we are ok dipping into savings to do it, or we want to spend windfall money on something we were thinking about because my wife worked some extra hours this month or we got some extra money from something, we finally have a concrete number of exactly how much excess money we actually have (or if we’re actually in the red and we cannot spend money on anything not in the budget right now!)
I don’t know if there’s a way to build this in to adjust automatically if a category gets added or deleted or to include/exclude additional accounts. My solution here is a little brittle because it uses a bunch of absolute references.
But it’s a start!