Guestimating is just fine. If you average 15mph on the road, estimate 15 miles for an hour on the trainer. Of course, it really depends on your intensity (which will affect your speed), but who cares, right?

BTW, I don't program my clients in miles. I simply use time. Mileage is such a variable depending on terrain, wind, traffic, etc. That makes it easy to record your trainer time. One hour = 1 hr.