I've flown trikes with as little as 1/2" or so clearance, but the occasional hard bounce
will eat up your props. I'd say shoot for an inch, if you can get it, and anything more is great.

At least as important as clearance, is getting the attitude of the airplane correct.
Shoot for straight and level, though a "nose up" of a few degrees won't hurt.
Some can handle a "nose-down" attitude, as well, but on some of my trikes, they
really wanted to stick to the ground when setup like that, and you had to horse em up.
With level, or an ever-so-slight nose up attitude, the plane will practically fly itself off.