The amount of set back would depend on how the threads were cut to a degree and how big the back of the chamber actually is. If there is a relief cut for the threads I would set it back at least the length of the threads. If there isn't a relief cut and you or a gunsmith feel confident you can get the cutter back in the threads then you can possibly go shorter. If it was me I'd cut the threads off and recut the threads to make sure it cleaned the chamber. I'd get a 7mm range rod to dial the barrel back in off the bore.
Brownells sells the stuff to do chamber castings. It's best to remove the barrel and then you plug the bore to do it.
I haven't measured my reamer but if I remember right it's suppose to be .512 right in front of the belt. So yours is probably at least .516 and I wouldn't be surprised if it was .518 or more. The amount of set back will change with the size of the chamber. I'm pretty sure a STW/8mm Mag is .512 at the base and .495 at the shoulder. So the more it is oversized the more you'll have to set it back to clean the chamber since the case has very little taper per inch. If the reamer goes in true with the bore it will be less than .001 over the reamer size.
I'm not 100% sure why it does it with a oversized chamber. I think it's because the brass extends beyond it's normal range too many times and becomes less elastic (work hardens). So upon firing it wont spring back off of the chamber walls. I would assume new cases are less likely to do it?