Rbeca, the rear roller on most tri-plex reels have reference points for the desired height of cut. Meaning to cut between a certain height, set the rollers on one hole, to cut at a different height range, the roller gets set in a different hole.
The purpose is to pitch the bedknife properly so the turf is cut in a corrent manner.
Once the height range is known, and the roller is set and level on the brackets, you have to square the rear roller to the reel frame.
Once the roller is square, then the front roller is used to held determine the height as now the pitch of the reel is correct.
I know of no manner in which correctly deteriming and setting the height on front roller equipped reels without using either a height bar or an accugage which will reach from the front roller to the rear with some sort of appendage that will lip on the bedknife.
Also, to correctly set the height, you first need to face the bedknife to return the bedknife to square.
Hope this helps