Thanks for the reply Keith. The odd thing is that α doesn't seem to be related to the angle of the vertical profile. Rather it depends on the DH.
Para 3 does say "With the approval of the Authority, the formula may be used with the actual approach slope and/or the actual length of the approach lights for a particular runway."
But why, on a 3 degree vertical profile, would you use 3.77 degrees in this calculation for a DH of 300 ft, but 3.00 degrees for a DH of 200 ft?