If they apply a margin of error of 13cm+ it would almost eliminate offsides at set pieces. Defenders would be at such a disadvantage that they just wouldn’t be able to play an offside line. That might actually be a good thing but it should be an intentional change, not a consequence of VAR.
Rather than just basing any tolerance on the hypothetical case of a Sane or Sterling running at full speed, they need to analyse a typical sample of offside decisions, calculate the average speed that the attacking player is running and use that as a starting point. Perhaps add a couple of cm to encourage attacking play. Whatever they come up will not be 100% accurate in all cases it just needs to be right more often than not.