How many groups do I shoot before I begin to make decisions?
Twenty groups gives a very good statistical basis for a decision, but if the standard deviation is small, I will begin to focus with 10.

I load around specific velocities that seem to suit each rifle best and account for the velocity variations due to trim length and seating depth so groups with the same bullet and powder within a few fps of planned velocity give a good indication of the effectiveness of the load. It doesn't take long to get a good sample for a powder bullet combination.
I load in 0.1 grain increments once I get honed in on a velocity that I want and watch to see how the accuracy increases or degrades as the velocity changes.

I load for 7 rifle calibers but tend to focus most on my most accurate rifles - the two Savage 6.5mm Creedmoor rifles that I cited in post #4, a Les Baer Super Varmint 1:8 twist .223, two Savage model 10 .308s, and a Savage model 11 .22-250.

I have also shot a lot with a CZ 527 Varmint .223 but it is getting old after almost 7,000 rounds.
I tried it the other day with the new 60 grain TMK and it performed pretty well so I am loading another 75 rounds to see how well it might do with another powder.

It seems I can always find a new bullet or powder to check out and using a rifle that you know well is a good place to start with a new variable.
I burned out a barrel with my original Savage .308 trying to find 'nodes' without really understanding the dynamics of the process and, although I kept good records, I wasn't an experienced enough shooter or reloader to take 'shooter and reloader induced variations' out of the process. After many years, I've had enough time behind the trigger and with a hand on the press lever to get that sorted out.

Now if age doesn't catch up on me too quickly - I'm 75 but still going strong - I'm hoping to get a couple of new rifles and continue the learning experience.