cool sketchbook dude, love all these studies and stuff you've got goin on. That master study looks prity cool too, one thing i would mention while looking at your screen cap is to keep your master study and your study at the same size. Its very tough to replicate a piece if your working from two different sizes, plus you loose alot knowledge of composition, shape, anatomy etc etc from doing this. Master studies are tough enough without making it even harder for yourself
Maybe hammer some black and white photo studies too to practice those values. Your doing great though dude, keep at it!