The placement of your hatch brackets is critical. Under say 11 1/2″ will not allow the springs to compress enough for the hatch to close completely; More than say 12″ will cause the spring/hatch bracket to interfere with the left side of the galley unit. I spent an hour or more establishing this as fact before drilling any holes.  I suggest the following procedure: mount your bulkhead brackets and put the springs on them and the hatch brackets on the springs. Find by trial and error the right length stick to hold your hatch up to the height that places one of your hatch brackets the prescribed 11 3/4″ from the hatch stiffener (market with blue tape). With your hatch propped up hold the first hatch bracket up in position and square and drill through its screw holes through the hatch. Temporarily bolt this bracket to the hatch and without removing the prop stick, hold the other bracket in position and drill through it. Remove brackets andncoat holes with epoxy.

The length of you arm should not influence where those brackets end up for the very important reasons stated above.