The most fuel you'll be able to put on a roo is about 120 ounces. 100 is normal and near average I would guess.

For a 500, you could probably get by with 80. I would suggest the two liter bottle method in the instructions especially for the 500. I flew mine with 104 ounces on a RAM 100 and I was getting 8 min flights. I then put a simjet 2300 on it and I'm getting 12 min flights same amount of fuel.

Sean