After reading some Bell and Garrett tech. I would say you need to figure out the efficiency or "effectiveness" of each cooler. I believe it was Garrett that mentions a/w being around 80-85% effective. Meaning 200* in would be 40* out, which seems incredibly low.
you'd need to know the outlet temp of your primary at max boost, effectiveness of interstage cooler, and outlet temp of secondary at max boost. You could use STAP as a baseline for calculations.
That's not how it works.
You're ignoring the biggest factor which is ambient air temp (Newton's Law Of Cooling), and your second biggest factor which is air flow (90% turbulent convection which is much harder to calculate).
I'm assuming they have some standard, or maybe there's an SAE one, that states testing at a certain temp in, internal flow, ambient temp, external flow, lighting (radiation), mounting (conduction), etc...
For that matter, assuming nothing would break/melt, if it was 1,000,000F at the inlet, and 0F ambient, your heat transfer would be through the roof because of the delta in temperature.
Specifying an IC is tough because the larger you go, the slower you are to spool and response goes down. Too small and you get hot air. You want a free flowing system, so to get the surface area needed to make an efficient cooler without building back-pressure, you make a ton of small passages and your inter cooler gets large quick. Another factor is what's surrounding it like a radiator then engine behind it to block wind and slow flow, or a crammed engine compartment that heat soaks.
The best thing you could do would be to test first hand. The second best would be to flow test your inlet piping to the intercooler, after the cooler, and then the cooler to make sure it's not the bottleneck. The cheapest and quickest thing to do is to buy off the shelf or copy a known working setup.
IMO a water to air is much more efficient if you can use ice water headed down the track.
Spraying an air to air would be a good alternative for track use.
And getting the most efficient, air to air cooler would be the best for street use.