I actually use a method that never fails. Put your unicycle at the edge of the stairset on the top, pedals horizontal, then drag it straight backwards however much you want into your mounting stance. Then, keeping the pedals the same, pick up the uni and movie it forward 6-12 inches to compensate for tire compression (the exact amount depends on your PSI, weight, etc., just mess around with it). Mount up and ride, and you should always be in the right position for jumping. Easy method to use, and it has never failed me.
uhm
hop in place on the edge of whatever, ride backwards, stop dismount, mount, have at it.
That works for a lot of people, but if you can’t ride backwards confidently (or at all, like me), then my method works fine.
Or my method. 
just get some “guts” and jump it from 5 feet behind it!
I’ve tried that. Didn’t care about measuring and jsut jumped when I had the chance to. Came up short, face planted. Didn’t get it on film, which sucked.
I either ride backwards or cheat. Hop up the stairs, and on the last one, ride out of it as far as you want to. Do a 180 and you’re good to go.