Yeah, I just used easy to calculate with numbers. And in the end, the combination of target size and how hard the target is to kill just gives you a figure for how much projectile mass per unit area that the target could be in you need - and for most reasonable numbers, you end up with dumb cannon projectiles being only really useful with <1 second travel times vs small and agile targets, meaning ranges of only a few to maybe 10 km even with rail/coilguns.
Of course, a real target, that can only thrust strongly in one direction and can only rotate that direction so fast, will not have a spherical volume of space it could be in, but rather a funnel-shaped one. If the target is coming towards you, that funnel is still going to project into roughly a circle, but with a much higher density of expected target positions towards the center than in the omnidirectional dodging case, which you can use to optimise kill probability vs ammo expenditure. But really in the end what you ideally want for point defense is high-power lasers.
Though there is I think one of the issues of hard-ish scifi space combat: If your setting's lasers are too weak, then you can easily fall into a situation of "the missile eventually gets through", and space combat is just sitting at long range lobbing missiles at each other until one party explodes. But if lasers are too strong, then they can perfectly counter any possible missile volley, and combat turns into lightseconds-range laserstar duels. I feel like making a tough scifi setting where guns matter is a really hard balancing act.
