What I have found is that a squad of 8x Oblivions has fairly good PD capability against the 30k km/s ASMs my foes like to use, but they get sandpapered to death by their AMMs at close range. My maths tell me that about 50x Oblivion IIs would be able to close the range, but that's a sizeable resource commitment and I'm still on the fence about whether that is the best option or if a new, larger ship would better.
Just don't do what I do and accidentally calculate your PD requirement using Gauss Cannon BFCs in your spreadsheet when your fleet has Railguns...
Nope, I am too lazy to do a spreadsheet for that. I use:
expected accuracy = tracking speed / missile speed
Which in my case yields accuracy = 0.07 (approx)
Then I have:
missiles destroyed = expected accuracy * number of shots per ship
Which in my case yields 0.07 * 16 = 1.12.
Missile volleys I am struggling with are about 50 missiles each with 70k km/s missiles. So I need about 50x Oblivions to counter that.
All of which I can easily calculate using my standards approved TI-30x non-graphing calculator! (which I hate... my TI-89 is so much more better)
EDIT: I neglected to include the effect of my BFC which imparts a 0.9 accuracy modifier on my 1.12. But that's not that important really since I need at least 50 Oblivions which is a huge investment to begin with. 60 would not be that much more. 75 is probably a better "safe" number.