Is there a formula/calculator to find the resultant compression ratio?
Not really in fact. Nantho's right here.
Well, if I can bring water to the mill, let's say that you have to think it in that way :
- Use the first compressor to catch the peaks with a high ratio (6 to 10 or even higher if you wish to limit) with a fast attack and release.
Set the threshold to get 1 or 2 db reduction : The goal here is to smash loud peaks and to still have enough headroom.
- Use the second compressor to make your track "steady" : with a more gentle ratio (around 2 to 4).
Set the threshold to get the amount of reduction you desire and use a longer attack to let the transients pass.
About the release, it depends on your track... Play with it until you find the sweet spot to your ears.
Teletronix is a very good one for this.
And yes, this technique is often used on bass and vocals. Hope it's a bit more clear.