basically look at the terrain and decide wat type of jump you want to build... whether it be a long lofty with little kick, or a shorter but higher amplitude one...
it basically works on the basics of physics which by the way i have never done... the the steeper the take off, the higher you will go but you will travel a shorter distence.
the best park jumps are usually curved so they slope gradually with a bit of kick at the end. its pretty much about finding a happy medium of height and lenght. its pretty hard to perfect it straight out..you need to constantly re-shape