Feeding angel fry is tricky, as any leftover food fouls the water. I feed mine bbs for the first 2 to 3 weeks, then start on finely crushed flake. As to the amount, it is a trial by error sort of thing. I hatch in a 10 gallon, and raise them in there for the first 2 to 3 weeks, after that transferring to a 20 or 29, depending on the spawn size.
To tell you the amount I feed I would first have to explain how I hatch & store bbs. I hatch one batch daily, anywhere from 1/4 teaspoon to 1 1/2 teaspoons daily. After removing the air supply & letting the bbs settle,
I suck them out with a turkey baster, and store them in an old 1 pint container. I store them, along with the brine solution, in the fridge.
They settle to the bottom of the container, for the first week I feed 1 standard eyedropper full 3 times daily. Starting the second week, I feed 2 eyedroppers 3 times daily, third week, 3 eyedroppers 3 times daily, and start feeding flake somewhere in there depending on the angels size.
What I've found helps is to add a small cory once they are freeswimmers. Don't add the cory before that, as the cory will eat the eggs or wigglers. I've used pandas, albinos, and smaller green corys, and haven't had any problem with them eating the fry. Corys eat any leftover bbs, they love it.
I've used the following chart (off the internet) as a guide to my Mimimum stocking rates.
Newly free swimming fry 40 fry per gallon
Two week old fry 20 fry per gallon
Month old fry 10 fry per gallon
Pea size bodies 3 fish per gallon
Dime size bodies 2 fish per gallon
Nickel size bodies 1 fish per gallon
Quarter size bodies 1 fish per 2 gallons
Silver Dollar size bodies 1 fish per 3 gallons
Potential breeders 1 fish per 5 gallons
Show Specimens 1 fish per 10 gallons
One breeding pair 20 gallon high tank
HTH, good luck, & have fun!