These distances are as follows. Just what constitutes a lock, and how one is achieved has also got to be figured out. If the ships are the same class, both move x toward each other. If firing at a ship of a larger class, the firer is moved 2x toward the target. And another idea.
Tractors simply dissipate kinetic energy.
This would be simplest. I'd restrict this type of tractor to capitals, and only one per ship. This would be my favourite option.