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.

The major reason I use a spreadsheet is just so I have cells where I can fill in all these things...BFC characteristics, weapon data, missile data, and so on. Re-doing the math by hand all the time starts to get annoying quick. I do even have calculation for the missile tracking bonus (which in turn requires you to input active sensor tech levels...and so on it goes).

Another potential problem actually is salvo overkill which can give you serious problems with excess leakers if your PD is marginal for the encounter.