I've never used the power planer to square a timber accurately. If a timber is out of square and I want to improve it, I will usually start with the 3 1/2" planer and plane down to a line, usually on one side of the face. Once I'm down to the line with the small planer, I would take the big planer and flatten out the face. If I want accuracy I would probably reach for the circular saw or the sawmill, or even the broadaxe which will take off a small out of square peice of face pretty quick.