Searching HN has failed me so...
I remember seeing an app that has a canvas, and on it you put individual 'sheets' or tables. You can reference between them as normal, drag and drop them around. The screenshots _may_ have shown math being entered too, I can't recall.
Because my calculations are made up of many mini-calculations this seems a much better idea than the normal Excel style of multiple tables on one sheet, as adding a row doesn't affect all the other tables in the sheet. And you can see them all at once, which you can't with multiple sheets.
I've checked out https://blockpad.net/ and https://soulver.app/ and I don't think it's either (based on their current websites). Blockpad is close but more linear.
Hivemind, can you help?
You arrange your tables diagonally on the sheet, such that no rows or columns of any one table intersect with any other table. You can then add/subtract rows or columns from any one table without changing the others.
I.e., layout your tables like this (hopefully the box-draw artwork comes through):