Different strokes for different folks....
Changing the charge isn't only about getting the most velocity, it's about finding the sweet spot for a powder and bullet combination that equates to the bullet leaving the bore at the optimum point in the oscillation cycle.
IMHO changing the length fine tunes it.
If you are stuck with changing the length to get the best accuracy your rifle can achieve you are missing out on another factor that can have a significant effect.
A change of 0.5gn or in some cases 0.3gn can change a 2" group into a .5" group which can then be tweaked further by playing with length.
If there is a dramatic tightening at 0.1gn difference then the powder/bullet combination is too finicky for me.
My advice would be to stop fighting with one hand tied behind your back.
I've had another thought....
I set up a rifle for a guy who had bought it brand new and the shop had set it up for him.
Regardless of how hard I tried I couldn't get any consistency from the damn thing.
Eventually I started from scratch with the rifle set up and found the bases hadn't been fully tightened down into the receiver.
Might be worth while checking bases/scope mounts/rings and action screws?