My PB Clyde mare, at 14 months old was 15.1hh and weighed 982lbs. I know she's not the same breed, but it might give you a rough idea. She was a good weight for her at that time - could feel rib easily enough, but not see them.
If your filly is 14ish hands then it's very possible that 600lbs might be a reasonable weight. Hard to say without seeing her.
I haven't read the book you have. What I found has worked best for our drafts was a premium grade hay, no grain mixes, and a mineral supplement (fed either free choice, or in a cookie I make for them)... though we do allow them to graze/forage year round - that diet might change if they didn't have the forage.
My mare was always well cared for - but my hubby's stallion wasn't. He'd landed himself in a bad situation and was very nearly starved to death. We tried all sorts of different feeds with him, and found that the simple diet of a high protein/low sugar hay, fed several times a day, with free choice forage was by far the best resulting diet.