Hello Jack,
First of all, sorry for not answering early and you had to figure out a solution by yourself.
I've seen that you have been exploring our API and I have to say that I'm really happy that you manage to pursue figure it out.
We are currently working on new features that will help to achieve your goal with OutSystems and not using JS.
From what I understood you have a calculated cell that actually depends on the values of other cells in the same row, like a sum, right?
With what is being planned, what do you think of an OS event that is triggered when a cell is edited and a client action that allows you to refresh a row with new data values?
Do you think we can reach out to you while we are in solution design and check if the solution would meet your requirements?
Thanks,
Bruno Martinho