Regarding creatures with more legs than two (dogs, dragons, horses, griffins etc). I don't have any clear cut answers for this one, but one method is to measure from crown to ankle just as with birds, another is to also include the forelimb in the body's length. The most important thing though is that the creature you're creating looks good and believable within the setting you have created and the style you have chosen.
Also check out Blue-Hearts' tutorial on winged humans, it contains a good wingspan calculation for humans and humanoids fav.me/d86yjqt
This might not be the most interesting reading material, but I think it's a good thing to know the basics before tackling more advanced topics.
The original plan was to do this and the other parts in the series as one large tutorial and tack it onto the previous series "The many types of avian wings", but I decided that it was probably better to split it up into its own multi-part series.
If you have any questions regarding wings or winged creatures, just ask me, I'll do my best to provide a good answer :3
The many types of avian wings
Part one, Standard wing types: fav.me/danpkgt
Part two, Aspect ratio and wing loading: fav.me/dao4odc
Part three, Wing types based on flight patters and foraging styles: fav.me/daq3zns
Creating winged creatures
Part one, Wingspan: fav.me/dbkv0l3
Part two, wing shape: fav.me/dcx1xpf
Part three, abilities:
Part four, extant and extinct animals:
Part five, mythological beasts:
Part six, creating your own creature:
Sorry, it's kind of fallen to the side, I just have so much that I want to do (which is a good problem), but now that I know someone actually wants a part two of this I'll try to make it a reality.
Equations aren't accurate since he is a four legged creature with wings and not a wyrm or bird, but it's a start!
One way could be to add the length of the forelimb to the overall body length, but perhaps that'd be a bit too much. (Maybe half forelimb length?)
I'm very uncertain myself, so that's why I haven't included it in the tutorial