[Fyron]

You could build the combat movement bonus into the engines. It would not be reduced until all engines are out, but it would prevent ships from being able to move once their engines are down. Of course this leads to the problem that it can not be tied to ship size. So, you can add a special engine with nominal strategic move and the combat movement bonus, and make it really big. Then use scale mounts for each hull size to get it to be the appropriate size for each hull. It would take up a bit more space on each larger hull, thus simulating a reduced effectiveness on larger, more massive hulls.