Thanks. Not sure how accuracy works.
So if final defence always fire at 10,000 km I'm looking for optimal or good range for beam PD guns/FC. So if range is like 40,000 km not sure how going even bigger range benefit at all. Range makes more sense when using area defence. If you choose max. Range 20k or 40k FC and turrets are smaller and you can install more of them than longer range turrets.
The hit chance modifier for range is:
1-(distance/BFC range)
If this value becomes negative, aka BFC range is < 10k km, the weapons will not fire at all. At 10k km, the weapons will have a hit chance of 0%. Being too cheap here will gimp your beam PD hit chance severely, as it is the upper limit of your hit chance against missiles. It can only get worse when the speed/tracking speed ratio gets added into the mix.
At your suggested 40k km BFC range your hit rate would be 75% tops, which is decent. If you want to use your PD as a secondary battery in beam engagements you could go with a bit more, but for me the size of the BFC depends on the type and size of the guns it is controlling. If you connect it with a or even two quad full size gauss @ > 2400 tons per turret, it becomes pointless to save 50 tons when it comes to the BFC. Just make sure it looks sensible to you.