How much high must be sets seatpost on the trial unicycle? Lower than at freestyle uni or?
Sorry on bad english…
really high, to the peak of the post length…
haha nah, it really depends on the rider, shorter than avg! i ride mine with some shortness to it, but wen its to short it screws up my sif hopping, so it theres a perfect spot for me!
it changes with evry1 so u just gotta go thru trial and error and u will soon find the perfect spot! start with it shorter than avg and work ur way up to a decent size for urself 
hope that helped