Configurations involving an "L" or a "C" layout should, as illustrated below, always subtract the depth of the adjacent heater cover (always 3") from the actual length measured.
For L SHAPE COVERS measure from the corner, to the end of the back plate, furthest away from the corner and subtract 3 inches, that is your length. ( example 75″ – 3″ = 72″ ). This needs to be done for both left and right sides. Your endcaps are calculated into all length measurements.