It is simple transposition
of formula, to which I have added some simple example values to prove the
maths.
If:
Convergency = Change of Longitude x Constant of the Cone
20 = 40 x 0.5
Then:
Change of Longitude = Convergency / Constant of the Cone
40 = 20 / 0.5
Or:
Constant of the Cone = Convergency / Change of Longitude
0.5 = 20 / 40