First, I try to find someone that is jumping it on a similar bike and ask them what they are doing, but since this is a newly constructed jump, you probably don't have that option.
I usually look at the jump and like you get a feel for the takeoff. I'll also see what it takes to jump to the base of the landing. If you can jump to the side of the landing, that will also give you a good feel. Using either way, you then have to plan on a little bit more to clear the jump. It's usually easier to overjump than to come up short. Then it's a matter of what it takes to jump similar sized jumps.
I would imagine that this will be a third gear pinned or fourth gear midrange jump on a 125. Just be careful and work up to it.