If you fab something like a boom pole, the height of the top link hole becomes important (the distance from the lower pins to the point where the top link attaches to the boom). The 3 point hitch is actually a mechanism called a "4 bar linkage". The tractor forms one bar, which is fixed; the upper joint of that bar is where the toplink attaches to the tractor and the lower joint of that bar is where the lift arms attach to the tractor. The remaining three bars of the linkage consist of the toplink (bar 2), the attachment (bar 3), and the lower lift arms (bar 4).
When I first got my tractor, I made a boom pole and didn't pay attention to the measurements. The geometry wound up wrong: I had made the attachment points on the boom too close together (vertically). Bar 3 was too short. The boom would tilt downward when I raised the 3pt hitch!!! Oops. Embarressing, since I am a mechanical engineer. So I laid out the geometry on Autocad with different permutations: bar 3 longer than bar 1, bar 3 shorter, etc. For a lift pole, bar 3 should be equal to or longer than bar 2....measure the distance from the point where the toplonk attaches to the tractor, to the point where the lower lift arms attach. Make the mounting holes on the implement equal to this dimension if you want the implement to remain in the same vertical plane (adjust the toplink as necessary). Make this dimension longer if you want the implement, like a boom pole or mower, to tilt upwards as you raise the 3pt hitch.
