Did a quick test and it seems to be calculated, but it's not 25%.
Some screens:
Here I'm facing the enemy and it shows 20:
When I present my full broadside it's 34:
So, 20/34 is closer to 60% than 25.
BTW, you may want to fix the Narvik Caper mission as it's supposed to be a covert ops yet it spawns you right on top (or should I say below

) of ASW ships.