I'm a beginner reloader and I've been thinking about some of the same things you are brought up.
My objective is to replicate/improve the performance of the Federal GMM 168grain HPBT. But use a Hornady 150 grain SP bullet. Why Hornady? Because I got 500 of them for free when I bought my Lock n Load AP press.
I know its going to be difficult because of the BC difference between the two bullets. So what I did is take a couple of Federal GMM apart.
The Varget powder is on the left and the powder from the Federal GMM on the right. I measured both of the federal and they came out to 43.9 and 43.8 grains.
As you can see the color is different but otherwise the powder looks very similar. Obviously I will be following the manual powder recomendations for the bullet I'm using, but I thought you guys might find this info interesting.