Powder: I use H4895.
Copper cleaner: I use Boretech Copper cleaner. I brush it in with an oversize (7mm) nylon brush. Let it sit an hour, then clean it out. You might have to do that a couple of times.
Bullet seating depth: mag length, based on bullet published length. However, my rifle likes Bergers off the lands 30 thou. I didn't try Bergers at mag length. I use Hornady 147 ELD-M now.
My rifle is also a .260 Rem: best 5-shot of 0.17", best 10-shot of 0.56".
If you can't get your barrel diamond-lapped, use Tubb Final Finish kit, complete process. By my estimation, it's about as good as hand lapping.