At least in Galactic Civilizations II, each turn represented one week of in-universe time, and each move action represented an approximately equal piece of that week (for a given ship/fleet, anyways; a fleet with 5 moves takes 1.4 game-days per move action while a fleet with 7 moves takes 1 game-day per move action, and a fleet with 28 moves takes 0.25 game-days or 6 game-hours per move action). These all seem like fairly reasonable time scales for battles; it's only really when the number of movement points becomes large that the in-game time scale starts becoming unreasonably short for a fleet action where neither side appears to be attempting to avoid the engagement.

As a result, unless Galactic Civilizations III either uses turns which represent a much shorter time unit or has significantly higher movement point pools than Galactic Civilizations II used, I don't really feel that the movement cost of engaging in combat needs to be tweaked. I suppose it wouldn't be bad to require a certain minimum fraction of a fleet's movement to engage, but on the other hand I don't really see that it's necessary.