Hi James.
Nice study, and one which as I know all to well will go on for years. Things to watch for is getting volumes, proportions correct, and not get too much into the details just yet. When doing such a study I highly recommend working on individual parts of the body as a study, this way each part gets it attention it deserves. I can see you have gone for the above average, a hero of 8 heads tall, in which id be inclined to put the 5th rib, the bottom of the pecs below the 2nd head high down, at the moment its above it which gives the whole torso a elongated look. Look at things like the Skeletal land marks, and make sure they are correct before adding muscle.
Does the clavicle angle back with a S curve in it?
Acromiom process sit in the correct place, and is the right size?
Look at the placement of the pit of the neck to that of the 7th cervical vertebrae, they shouldn’t line up, 7th should be higher…
Look for the 6th,7,th,8th, ribs, and costal cartilage, they are important because the 5th is almost in like with the bottom of a male chest, the 6th, 7th. 8th are where the visible portion of the Serratus Anterior, and Thoracic portions of the external oblique are.
Look for where there is plane changes in the rib cage.
Look at which order the insertions of the Latisimus Dorsi, Teres Major are, as well as the order in which the back muscles lay. Also include the arms in this.
The Extensors of the forarm, look at the origin and insertion of the brachioradialus in relation to the ridge of the humerus.
Locate the three positions of the pelvis, Anterior superior Iliac spine, Posterior superior iliac spine, and peek point of the iliac crest.
Look for the correct location of the Greater Trochanter in relation to the pubic bone.
Define the different forms of the patella to the patella ligaments, to the Tibial Tubuerocity.
How does the Patella relate to the location of the three heads of the quadriceps.
What to take from this is go back to Skeleton, and really look closer at portions, and relationships from bone to bone which includes the angle, plane changes, scale, length. If the Skeleton is correct, then the Origin and insertions will be correct, then you can look at forms, plane changes, and relationships between the muscles. Keep going, and keep enjoying it. its a huge subject, and gets more involved as you go. I dont normally reply to these threads as it can take forever, but I wish I had someone comment on mine years ago in which could have saved me some time. I highly recommend joining https://www.uartsy.com/ there is a Artistic Anatomy in Zbrush course which will take you a long way, and is priced very fair. I also recommend Scott Eatons Anatomy class, its not cheap, but its very comprehensive.
Danny.