No great answer here, other than that is why the "lone ranger" exists. The "lone ranger" is the auxiliary down box on the pressbox side of the field. He holds the previous spot until the ball is ready for play for the succeeding down. If there is no "lone ranger," there are still a couple of tricks available. Try to get one of the chain crew to track the succeeding spot, by placing a long piece of athletic tape on the down box, or one of the stakes, and write down the yard line for each succeeding spot. That could be done by the clip guy, too (with a clip board or small note pad). If they are really good, they'll track the time, too, i.e., each time the clock stops until the next snap, they'll write that down, in case the clock gets started for the next down, but there was a late T/O request that got granted, or false start or something. Finally, I have heard of box guys that drop a bean bag at the succeeding spot, just for that reason, but that means he has to pick it up each time he moves; it could be done.
Don't depend on the line judge. The might happen to be at the previous spot, if he doesn't move. Under most circumstances, yeah, he might 'hold' on the NZ for a while, then move to cover action as needed. But, if he is camped on the NZ the whole game, he ain't being as much help as he should be. Don't hang your hat on that.
The lone ranger is the best thing. Otherwise, yeah, it may be the best guess, and that won't be fun.
Robert