The Preston Blair is a great book to start with.. The cartoon figures aren't terribly complicated. But yes, it's important to try and draw them as closely to the way they're drawn, as you humanly can. Not because you're training your hand to copy the linework. But, because you're training your Eyes to properly "see" the shapes and silhouettes.. Which will eventually allow you to draw them better..
Sounds like you're getting the hang of drawing things to scale, and that's good. But if the quality of the shapes isn't matching those in the Blair book.. then that's a problem. To accurately replicate the images in the book, you'll need to accurately draw the shapes.. and especially the proportions.. found within the book.
My general advice to doing this, is to force your Brain/Eye/Hand to draw the Negative Space silhouettes found between the Legs and Arms, etc. Trying NOT to draw the Positive Space that the Figure itself occupies is initially difficult. But if you find ways to "measure" using what the Figure is giving you, sometimes you can come up with a more accurate drawing..
Occasionally, if the figure is clothed you can also concentrate on the distances that folds or buttons have in correlation to Arms/Legs/Shoulders/ etc..
The trick is all in the "seeing".. That's the part you have to work on. It's relatively easy to "ballpark" Head volumes, etc. But getting all the pieces on the Figure to fluidly coincide with each other, is the trick in re-creating an accurate copy of a Form or Figure..
Hope that helps you! Good Luck,