I don't know the actual mathematical formula for calculating excursion, I let WinISD do it for me.
The max. power chart will tell you how many watts it takes to reach xmax. You can up the power and find out where it reaches xlim.
As for broadband spl....double power = 3db and ten times power = 10db.
So, disregarding frequency response for a minute and using an average spl figure....lets say your driver is rated 97db @ 1 watt. Doubling driver count gets you 3db so your 2x cab would be an average of 100db @ 1 watt.....so that means 110db at 10 watts.....120db at 100 watts and 123db at 200 watts. I stop counting at 200 watts, beyond that, it takes a whole lot more power for very little additional output. For example, you'd need 1000 watts to reach 130db and this is beyond what most drivers can handle. Vd, xmax, etc. dictate output in the low frequencies.
Somewhere up there, there is some physics thing where the system can't be made to go louder no matter how much power you throw at it.
Those numbers are for a measurement 1 meter away from the cab. Ignoring room effects (like playing outdoors/measuring in 1/2 space) you lose 6db for every doubling of distance away from the source. So....if you're at 123db @ 1 meter, you get 117 db 2 meters away, like on stage 6 feet from the rig, 111 db 4 meters away, like the dancefloor in a small club, 105 db 8 meters away like the nearer audience in that small club, and 99db 16 meters away like the folks further toward the back in that same club.
Those are raw numbers for figuring output, of course, room effects, frequency cancellations, eqing, bodies soaking up sound, etc. all come into play and you're likely not up in the high 90's-100 db in there or you'd get asked to turn down.