Very well, that by itself is fine, but you do also understand that this makes naval mines largely useless correct? Because if I put 30 mines in one spot, the most they can do is destroy one ship according to this.
Maybe don't put all the mines in the same spot
If you stagger the location of the mines, that will also stagger their arrival at the target. For example, if you have missiles that are 20,000 km/s, set up one set of mines at 99,000 km and a second at 101,000 km (and a third at 201,000, etc.). Or setup mines at max distance but in two or more directions, so that ships moving away from the jump point will be hit by one and then the other. Or set up mines in concentric circles.
I could also perhaps add some form of random targeting if a second stage missile is launched without a parent fire control.