On a Lambert Chart:

Convergency = Change Longitude x 'n'

'n' = Constant of the Cone or Convergence Factor, call it what you wish.

Change Longitude = Rakovnik 013º 41.5' to Frankfurt 008º 38.3' = 005º 03.2'

You will note that the two positions are both located at 50º 05.9' N, therefore the Rhumb Line Track following this line of Latitude from Rakovnik to Frankfurt is Westerly and therefore 270ºT.

However, the straight line track, i.e. the Great Circle Track, is 272ºT.

The difference between the Rhumb Line Track and the Great Circle Track is Conversion Angle, in this case, 272ºT - 270ºT = 002º.

As Conversion Angle is 1/2 Convergency, Convergency must be 2 x Conversion Angle. 2 x 002º = 004º.

We now know:

Convergency = 004 º
Change Longitude = 005º 03.2' (005.053º)

Put these into the formula:

Convergency = Change Longitude x 'n'

004º = 005.053º x 'n'

Transpose the formula:

'n' = 004º / 005.053º

'n' = 0.79