You have to wrap your mind around the fact that space warfare is 6 dimensional. X, Y, Z positions and Vx, Vy, Vz velocities. The set of possible future positions of a spacecraft is an opening exponential horn. (Actually, something more complex than a horn.)
You want to set up a spreadsheet with the speed, distance and acceleration equations.
You need to decide how long the powered flight envelope is for your missiles. In Weber's Honorverse books, missiles at the start have an acceleration of about 8,000g and a burn out time of about 3 minutes. This gives them a range measured in millions of km. Later in the series missiles improve. In general the armaments race makes an interesting thread that runs through the series.
Weber doesn't use the free flight option much. But consider a weapon that was smart enough to be left behind, or to circle around the target. Or consider a missile that could be retrieved if it missed the target.
You need to decide how smart the missiles are. Do they have frequency shift radar so that jamming is harder? Tagged pulses so that each missile knows it's own radar return signals. A somewhat lighter version that comes in ahead of the pack as electronic jammers for the slower main bombardment. Do they have bomb pumped x-ray lasers to increase their kill radius.
Much depends on the electronic counter measures. E.g. When a rail gun throws a slug, is it coated with a radar absorbing coat? How far can the enemy detect the magnetic pulse from firing? How far can they detect the shell? Do rail guns throw explosive shells? What is their radius of effect? Same as missiles?
Missiles are easier to detect due to exhaust plume. Seeing one gives you an aiming point for tracking radar. But consider a missile that is restartable. It accelerates for 20 s, at 650m/s so it's now going at 13 km/s At this stage it throws an asymmetrical cloud of objects that have the same radar cross section. Suppose they are thrown sideways at 1 km/s. Now your target ship has to figure out which one is real. While the target's tracking computer is having fits, 2o seconds later the cluster of decoys is 40 km wide. In that same 20 seconds your missile has traveled another 260 km. Now only 2 seconds from the target, the missile restarts its engines and makes the final dash.
So missiles have a target envelope measured in hundreds of km given 20-30 seconds burn time.
The Vulcan Phalanx on the aircraft carriers throws shells at an incredible rate, and fire director radar tracks the outbound shells to refine the solution. It fires 90 rounds per second at velocity of 1.1 km/s so for a target 2 km away there are about 160 rounds in flight. It uses a 20 mm shell. This, I think, is the role of your rail gun. It targets incoming missiles and attempts to destroy them at a distance of about 20 km. If it could fire at, say 5 rounds per second using amount the same size shell that WWII anti-aircraft artilery used, but vary the muzzle velocity slightly so that a 4 second burst arrived on target at the same time. In addition the final aim would be modified so that the coverage included the space where the incoming missile could evade to, they would explode at the same time, creating a wall of shrapnel.
Rail guns don't matter until you are very close. If the kill radius of the warhead is 10 km you don't want it blowing up any nearer than 20 km to yourself. So if you had an optimum engagement envelope for RG of 40 km that means that it has 8 seconds travel time. Target ship has to change it's path by 10 km in 8 seconds. That would require an 8 second 12G burn at right angles to the present course. Pushing it.
But if the cycle time of an RG is 5 minutes, then it becomes a game of tracking when the opponent last fired. 300 seconds at 5g (50m/sec2) is 1.5 kps. In that time 1/2 at2 = 25 * 90,000 = 2400 km. Lots of time to get close enough
I don't think rail guns matter initially.
Are rail guns on turrets, like a modern battleship, or are they essentially fixed, like the bowchaser on a 1800's frigate.
Railguns may be better used as an anti-missile defence.