Unless you are loading them one at a time your magazine plays a big role in your overall cartridge length. Let's forget about AR10. This is just example. Say your tikka is accurate with the bullet .006 off the lands, which puts your total cartridge length at 2.91 But now your overall cartridge length is longer than 2.8", so they will not properly fit in your magazine. But if you seated the bullet another .003 they would the feed properly from your mag. In other words your magazine dictates the length of cartridges that you can run in that particular rifle. Accuracy may be better .006 off the lands, but your magazine may only work if you seated the deeper by reducing total cartridge length by running .009 off the lands.

The amount of powder is also dependant on each particular rifle. Load data tells you for example a starting load of 42 grains, max load of 47 grains. So you start at 42 grains and work up to 47. When you start getting to the top limit you check for pressure signs. I like to load every step in .5 grains to find accuracy. Generally there are 2 nodes that will group tighter than others. Once you find them you can then get more tedious. Hopefully I did not just confuse you. I'm not good at typing shit that makes sense. But honestly it's not difficult at all once you get into it.
I buy powder from dsh. I buy bullets from Kevin's.