VAR doesn't look at whether it crossed the line - that is goal-line technology. Seems to work fine.
The problem with offside is that it can say whether it was or not, but people complain that it was only a toenail etc. It's either offside or not, however tight, or why use it? Even if there was a margin of error, it would not make any difference, as if you gave say 50mm leeway, what if it was 51mm?
I really don't know the answer.