If you wanna ride seat in trials you have it pretty low, if you wanna ride SIF (seat in front) trials you wanna have the seat, as Danni said, slightly above crotch level. Try and learn SIF trials first and if you don’t like that style you can cut it down even lower to seat in trials.
Here is something you can try and it works…
Measure your inseam and set the height of the seat to your actual inseam. Measure from the bottom of the crank to the middle of the seat, not the top. That would be an ideal position to ride long distance. If you don’t feel comfortable, you sure have a good starting point.
For trials start lowering from this height until it feels confortable!