I hate using WW2 parlance but everyone knows what I'm talking when I use them to classify ships:
Battleships, freighters = 2
Battlecruisers, cruisers = 3
Most escorts = 4
I don't want them blasting around too fast (I'm expecting a practical in-game velocity of 2-3x thrust) so I've decided to 'cap' velocities at 20". Perhaps that is the speed of light, which is the maximum matter can move at.
I'm going to penalise fast moving combats. This will make high thrust craft hard to hit, which makes sense.
Any combat over 10" velocity will be penalised. This is Relative Velocity, so two ships heading towards each other at 5" and 6" velocity are already over the 10" limit.
Stationary target = +1 torange
Velocity 1-10" = 0 modifiers
Velocity 11-12 = +1 to range
Velocity 13-14 = +2 to range
Velocity 15-16 = +3 to range
Velocity 17-18 = +4 range
Velocity 19-20 = +5 to range
Velocity 21+ = no combat possible
This should keep speeds manageable whilst adding tactics to the velocity and angle of approach.
The extra measuring to find velocities is a pain in the butt though. Viva la hex map!
Firepower : Defence Ratio (or - how fast things go "boom")This is very situational, as speed and thrust allows ships to avoid concentrated fire.
I would expect a battleship size vessel to cripple or possibly vaporise a stationary destroyer, but be flat out doing damage to it in a high velocity battle.
Hopefully the fact thrust and size can be used as damage avoidance will help small ships survive:
a escort (+1 small) using 2 thrust to evade (+2) would be hit on a 8+ at 5"
a battleship (-1 large) would be hit on a 4+ at 5"
Thus at what I anticipate to be a 'normal' engagement range, a escort would be 2x harder to hit.