Watch people or if someone is nice enough to let you follow, do that. 15 feet and even 30 feet are pretty small, so there is really not much risk involved there. When I go to a new park I will usually follow someone who is throwing down after I ask them.
At my home mountain though, I'm so comfortable with our jump lines that I can just drop in any day and hit the sweet spot. Just takes practice and you'll realize how a jump looks and how that translates to the speed and how that feels.
"For me, my family, my friends and skiing, that is for me. That's my life. The joy I get from skiing... that's worth dying for." - CR Johnson