You take what has been reckoned by scientists to be the margin of error and expand the lines by that much each way. Attacker back, defender forward. If they overlap at all then it should be classed as level and onside.If they don’t then offside with the error rate already accounted for. Yes you would still be using technology but the margin for error would already be built in so less chance of very close calls being wrong. The previous principle was the attacker should get the benefit of any doubt in close calls after all.
I would still scrap var as it’s killing the game though.