It's not too hard. You just have to imagine all situations, like I said: the leg being stretched out. Our perhaps flipping the picture around. That usually helps. But the best thing you can do is use a skeleton!
A skeleton? Like a line system used to check for geometric equality in perspective? Also, what's with the exclamation mark? Yelling at me or something? lul
Also, if you wanna argue about proportion, look at the wings, they don't connect to the body, they're improperly sized (too large for her body) and they don't work at all with the angle and curvature I wanted them to.
An artistic skeleton can be anything, but for ponies it usually consists of circles or bean shapes. I also occasionally use lines to symbolize legs and tails. When things are simple-ized it's easier to correct mistakes in proportion. If you want though, you could measure the lines. I'd say that's going too far, but it's probably a good practice I should have taken.
Skeletons are easy to erase if you draw them lightly, and oftentimes you don't even need to erase them because they become part of the pony. Since I can't show you a picture, you'd just have to search for a pony skeleton. But I'm just assuming you already know, because you're a great artist! And no, I wasn't yelling at you XD It was an exclamation point of joy!
Also, I think the wings look pretty great. Big wings are awesome even if they aren't canon. The only problem is that her right wing is going back and her left is going forward. I make that mistake too, though, so I hardly even noticed.
The best thing you could do for her long leg is take off the "hoof" and make the "shin" her hoof. Don't mind me if it seems like I'm talking down to you or something. I'm not, I'm just awful at presenting information I think you might like to know.
I already know what that is, just never heard anyone refer to it as a skeleton. (I normally just say guidelines.) The reason it's disproportional is I initially had it going down like the other leg, and then I changed it up with out testing the guidelines to see what would be proportional before hand.
No, I'm not a great artist, I'm a "Self-teaching-student". I study how to draw tutorials and realsim to figure out what works and how to go about drawing things out of my comfort zone.
Also, your advice on "break the hoof and make the shin the hoof" wouldn't work, that breaks the rules of anatomy. How to fix this would be to shrink down the proportions of every "bone" by a small portion. Therefore keeping it anatomically correct and fixing the proportion error.