I want to avoid instances like the FA Cup semi at all costs.
When you have a law like offside, then there is no subjectivity, as you are either off, or you are on. But when you bring technology into it, you are giving decisions where it is so marginal that it does not matter; millimetres.
You can say that if the margin is so tiny, that they are functionally level. No advantage is gained, and it does not serve the interests of the game to be pedantic over such irrelevant margins.
I assume that a semi-automated technology would do this?
VAR use for offside is one of the worst applications of it in the game. If you're going to use it, use it like the rest; to overturn obvious errors.