How long should the tube be for a ping-pong ball launcher?

[sciencewiz]

im trying to build a ping-pong ball launcher and i can't figure out how many degrees to elevate it or how long to cut the tube. has anyone done this before and if u have help me! also if u have any ideas that woud be extremely helpful as well. if u help me with mine ill help u with urs.

 

[sciencewiz]

also it has to shoot 12 ft into three coffee cans from the launcher table. if i cut the tube I am going to launch it out of 1ft 6in long & elevate it at a angle 6in off the table wound it reach the cans on target?