I wouldn't worry about percentage. how are you ever gonna know? Is it percentage based on volume of food? Based on weight of food? Its unnecessary.
A big salad every day and an insect meal 2 or 3 times a week is what I've been doing for many years now with good success
The 20g might be a bit small for an adult, even as a temp.
You can go to a big box store and pick up a decent sized plastic tote (like what people use for storage) for pretty cheap. That will give him more room than the 20g will for pretty cheap. You'll just need to buy another uv setup and heat setup.
But I do think you should separate until the issue clears up. Just incase it is fungal, you dont want it to spread to the other dragon.
Sorry I guess I forgot to respond to your health thread.
-Brandon