Ready, Aim, Fire!

http://www.virtualtrebuchet.com/ Simpulator with mathematics

for a simple design, and adapt that to your dimensions. But reverse the direction of shooting, that is, long end of the frame toward the target and arm down on the short end. Look at the pics to see what I mean.For a limited size trebuchet, it's a good idea to "caulk" the arm as steeply as possible. I'd recommend an axle height of around 16 inches and a beam with a long side of 18 inches, and a short side of 4 1/2 inches. Use about 80 or 100 eggs worth of counterweight, somewhere between 5 and 10 pounds. A heavy beam will kill performance, so use something light but adequately strong. The sling should be about 80 to 90 % of the long arm length, measured to the pouch center (you are using a sling, right?). A golf ball would be good for practice: more reusable than an egg.

Don't make yourself crazy over the frame. It's just a bracket to keep everything aligned. Check out the Little Dragon trebuchet

http://www.stormthecastle.com/trebuchet/how-to-build-a-trebuchet.htm

https://www.instructables.com/id/Cardboard-Trebuchet/