Dragon? Only possible with Magic? No.
Dragon: big, strong, hard-to-kill, flies.
Marden: 25% of total mass IS dedicated to flight. Problem!
Hard-to-kill? Small hitbox. All organs in ribcage, even a copy of the brain.
Big? 450-500kg. Mostly air, helps breathing and storage for breath weapon. Still as large as a bigger horse. Horse only small on monitor because monitor small already.
Breath weapon? Swarm of micromachines.
Strong? Strong but tires quickly, every muscle is producing force, not much space for fuel on board. Bites really hard, tail can whip and shatter your skull.
Sorry for going Unga-Bunga, but I had written these points several times already in different questions.
So, how does a dragon use their limbs anyway?
In combat, dragons have two speed-dices. They can freely use the passive "Presence" ( D&DD1 2E; PHB, 82) to force a given emotion on every character of their choice. After that, they usually make a "bite" attack, followed by a "lunge" attack where they move one square ahead; and whoever is in that square, if they fail their dodge roll, they take massive slash damage and are knocked back by one square.
So, dragons tend not to use individual limbs to make multiple weaker attacks and opt to use kinematic chains instead. Most of their power is in their spiked tails and jaws anyway.
During take-off, dragons pole-vault into the air, using mostly their wings and flight muscles, and their hind legs to some extent.
When on the ground, dragons are either trying to beat Usain Bolt's world record or walking, no in-betweens. They are good climbers (sharp claws and gecko-like paws could help with that) and are also able to crawl and swim. Well, they can use their tails to assist with the latter. As for climbing, they live in caves in mountainous areas, for a point of reference.
You gotta carry that weight, as dragons aren't a good alternative to beasts of burden. They can carry a Mk 47 Striker and about 30 HE grenades for it comfortably.
That's about all. So, I was having a relatively straightforward issue. Even with strengthened tendons and bones (carbon nanotube "magic"), there's nothing I can do about the muscles. Dragons don't need much, but if they can't haul their own weight without panting like crazy, there gonna be problems.
What would be the minimal weight (in percentage of the total body weight) for the four limbs (back and forelegs) and the musculature that moves them, that passes my functional test?
When I talk about related muscles, I mean the ones whose insertion or attachment point is one of the leg's bones.
1: Dragons&DungeonDelvers (well, it's my bepis to WotC's pepsi, now with 20% more Library of Ruina)