well if you wanted to just jump a big ramp that would be easy to figure what you need to do. But you want to do a counter clockwise backflip? How do you plan to flip that thing around? That would be incredibly difficult thing to do. If it's actually physical possible(i don't think it is) what you need to figure by experimenting it how much time you need in the air and the distance you need. After that you can use these two equations to figure out your initial velocity.
R=Vi^2/g sin 2(theta) //this is for range the other person's
// like one person said you'll probably
//want to be at (theta)= 45d
//so just set it to 45d and solve for Vi
T=2Vi/g sin (theta) //For the amount of time you're in the air.
You'll simply have to figure out these two things and then solve your initial velocity. T and R depend on how fast you do your backflip which you can only figure out by experimenting. By the way I have doubts you can get enough velocity from a 4wheeler to get enough R and T for your flip. I suggest you set the ramp going off a cliff to get maximum flight time! :D
Actually thinking about it, you'll want to do something bigger than 45d, going up to about 70d. You should be able to simply lean back and use your weight and let the bike fall back over you. That's the only possible way I think of doing it. If you do lower you're gonna have to put A LOT of muscle in to it to throw the bike off it's center of mass. Which is very risky. good luck.