You can nail it to within 1 mph by using speed trim instead of trying to read the instrument where the needles are 5 mph thick..heheh
Just type .speed xxx (<---whatever you think the speed is)
Then switch from level trim to speed trim. If the plane climbs, you picked a speed that was too slow. Hit level trim again while you type in a faster speed and try it again. Keep in mind that the speeds you type in are the indicated speeds, not the true speeds. Likewise, if the plane dives, you will need to type in a slower speed for the next attempt.