You really dont have enough time to think about it.
I just usually am on the throttle so i dont UNDERjump or case it or people will run you over or you will be hurt like hell(if you case).. Its better to over jump than under plus after riding the track you know about how much by instinct.
After you've done a lot of jumps that are less dangerous, (singles, tabletops, etc.) you can get a pretty good idea of how to time it correctly. Also, like tn motox said, it's much better (most of the time) to land too far than too short. Being able to do double jumps comfortably comes with experience and the more time you spend doing jumps, the easier it is. In addition, after you've done a jump once, the next time it is exponentially easier to do it a second time. Take your time and work your way up to the big stuff, ride safe and have fun!
[This message has been edited by Ace McCool (edited 03-08-2001).]