As far as correct preload without a wheel, I'd just keep tightening the top cap, wiggling the steerer to make sure things are seated, until you can no longer twist (steer) the fork blades easily with your fingers, and then back off until it's clearly turning without any binding. That should get you close. If you'll continue to use it on a trainer, then the preload isn't all that critical anyway. If you eventually take it back to the road, you could double-check your preload using your usual method of rocking the bike with the front brake engaged.