How to determine dB headroom for Woofers

Yes. like a 2.2.

2.3 is better. 2.4 is better still. You get the biggest benefit going from 2.1 to 2.2, a little more but not as much going to 2.3, a little less going to 2.4. Adding low frequency sources is better but at decreasing returns. You can add later if you aren't happy with fewer sources.

You won't position those low frequency sources like your two main channels. When the room is modal you aren't listening to sound that travels in a straight line. It travels between the walls from many different directions before you even understand what you've heard. The direct line between you and the low frequency speaker is totally irrelevant unlike the higher frequency spectrum where you position the speakers like normal.
 
I have played around a little bit with room simulations but mostly subwoofers and placement of the speakers, but I will look in to it and see if I can make it work.
I should probably start using the microphone I got over a year ago and see for myself. lol