Your on the right track. I'd think 1:9 or 1:10 would be optimal. I've got a heavy barrel Mini-14 in 6x45, it's a little different animal than what your putting together but as someone who's played around with this cartridge a lot (and the 6mm TCU) I do have some advice.
You'll want every bit of length you can get in the action. The key with the heavy bullets is to load them long (something I can't do in my Mini). Nothing says you have to stick to the .223 cartridge length of 2.100". With long 105gr 6mm bullets you're gonna want something around 2.350" at least. If you don't load long the bullet will steal powder capacity and velocities will be poor. I don't know how long a cartridge you can get into a mini-Mauser action, if it was less than 2.400" I'd strongly urge you to go to a short action receiver.
Consider the 6x47 instead. It's a near 6x45 twin but uses the .222 Magnum case. The extra 2 -3 grains of powder capacity might seems minor but in actuality it affords a significant velocity boost. A "Magnum" in name only, .222 Magnum brass is not hard to come by. In the '60s to early '70s the 6x47 ruled the benchrest scene. It was dethroned by the short & fat 6mm PPC.
Play around and load up some dummy cartridges yourself. Neck some .223 brass to 6mm and stick bullets of different length in so that the base of the bullet is even with the shoulder of the case. Take the longest one you're likely to shoot and give it to the Smith who barrels the rifle to use as a guide for determining how much free-bore you need ahead of the chamber, you'll want around 0.010" to 0.015" lead for best accuracy. -- Kernel
