Sum of all cells in a column where the next column in the row is blank

What I’m trying to do

I have a table showing the hours of TOIL I’ve accrued at work recently. This is arranged so each row is its own instance of work done out of hours, with the column headers:

| Date | OPP/Ticket | Time worked | Hours TOIL | Used on |

I’m looking to get a single figure for hours left to use, ideally positioned separately above the table. The maths for this is really easy - add all numbers in column “Hours TOIL” where “Used on” is blank.

Things I have tried

I’m a little unsure where to start to be honest. My Google-fu is failing me slightly.
I did find this thread on the forum which looks like it should be modifiable to do what I’m after but I’m not massively understanding how it’s approaching it: Sum, group and conditional calculation - Help - Obsidian Forum

This thread is also heading in the right direction, but looks like it’s making the total its own table? I’m not specifically against that, it just feels a bit odd: Dataview Table: Summing a column - Help - Obsidian Forum

Any help would be much appreciated - I’m mostly hoping for a better understanding of what’s going on here and how to approach this issue so I can do it myself in future, this is a fairly typical thing I end up doing in my notes.

To help you understand, and guide you further it would be nice to see what your query looks like and what results you get.

Without it we’re close to guessing on what to suggest…