You've received some good advice and your number of .035-.045 freebore will work well.
If it were me, I'd go .040-.060 freebore. There's no specific number that is an end all, be all here. You just want to keep the bullet above the neck/shoulder junction, have plenty of grip and maybe a few thou to chase the lands. I'm just saying that I wouldn't get real hung up on the exact fb amount, as anywhere from zero to .100 will work with the long neck of a BR case. You're in the right range for the light bullets. I'd go with a .272 nk.
I have found powers a bit faster than Varget to work best with light bullets in a 6BR. N135 has been a staple for years and don't overlook H322. Some of the best 6BR's I've ever seen preferred it over all the rest. Stay between H322 on the fast side and 135 on the slow end. The 6BR is easy to load for. Several powders in that range can give very good results.
Generally speaking, it likes pressure and the bullets seated anywhere between .010 off and .010 into the lands.
I began my benchrest shooting career

with a setup similar to what you're building. It was 14 twist Shilen Ratchet and would give PPC's fits with now discontinued 74gr Bergers. I used that bullet in a few different 14 twist barrels and it shot well in all of them. It's a shame that Berger discontinued it.
Much info you'll find is related to shooting heavy bullets. I've bought a few 6BR's that just wouldn't shoot for the sellers. They were all using Varget! I'd buy the gun, load n135 and/or h322, and their guns instantly became shooters! Varget is great with the heavy bullets in a 6BR but I've not seen the first one that liked it better than the afore mentioned powders, with the light bullets. That being 60-74 grains. YMMV --Mike