Just used this complex formula in comparison to Muppet's and Bytjie's on Excel to test bonuses on my new map:
A is the Surrounding Borders (Not Connections)
B is the Territories in the Bonus itself
C is the Borders that Connect to other territories outside the bonus, (Borders you'd have to defend in the bonus)
The *100 and *90 are just to give C less weight. This should give you a rough guideline of the bonuses, also take in to account the bonuses values nearby, and ultimately decide yourself. :D
Mine, Muppets and Bytjie's all work well, if want an even better number, take the average of all 3 formulas.
Haven't tried Moros's though.
The areas the averages of 3 highlighted on my map, were that:
Belarus needs to go up to 5,
West Ukraine up to 5,
Southern France up to 4,
England and Wales go down to 3,
Iceland down to 2 (It was 2.44)
Northwestern Africa down to 3.
Some of these will be changed now, the formulas were very useful. :)