I think you have most of the measurements figure out, now is just applying them and keeping it consistent.
Heres a red line to kinda point out what im saying.
So Diagram (A), one of your practice shots, I think you got a lot of things going on here, most of these are correct proportion wise, not prefect and there are still a lot of errors but if we just look at the figure alone ignoring all of the muscular structure, I think you got the silhouette pretty good here.
Diagram (B), Height proportions are good but around the joints and the limps, they are way off. I think the reason why your 8 heads seem too long is because you thin out the body too much. This used to be one of my weak points as well. You just gotta widen the limps, especially the arms, and be careful of those overly thin joints.
(C) is just a redline, and (D) is my draw over showing the proportion of the body as a whole. So yeah, just try to be more aware of your weak points in your next sketch and focus more on those areas. Heck, maybe you should just focus on one part of the body and draw a bunch of it like so
Either way, KEEP IT UP! 8D