A long time ago, I used Pluto as an 8" Planetoid to develop Board Scale, and at that scale, Earth is almost exactly 48" wide, so I think it's a good Scale for distances. However, at that scale, a game Turn must be a very long length of time so physical projectiles can travel that far in a Turn. Then again, maybe Torpedoes can reach a reasonable fraction of c in a short period of time, and the scale is large while the Turns are fairly short.
It might make more sense to figure out a time scale per turn you like, and then figure out Board Scale based on the distances.
Hey, where's that sub-mariner to tell us how long a modern ship-on-ship combat is expected to last?