Just to satisfy our (and your) curiosity, take a long bullet, say a 220gr, and seat it backwards in an empty case, then seat it progressively further until the bolt will just close on it (use bolt force to seat it the final step. Measure the amount sticking out of the case and that's your freebore.
If you leave it in 8x57, just seat bullets out so that they're flush with the bottom of the neck. If that's too long for the mag, spend a few bucks to get the mag lengthened, but while you're at it open it up to 3.400" or better, so that should you rechamber, you won't have to do it again.
Lots of guns shoot very well with lots of leade and freebore. Problem is, if it doesn't shoot well, you can't necessarily blame it on that.
Me, I'd go to 8x63, but I have lots of it, otherwise I think that 8mm-06 is the ticket, as dies and reamers are cheap and available.