I think that treating it as adiabatic is a reasonable approximation. If I use the wiki on adiabatic process example again, the result using a 10:1 compression - expansion will be a simple reversal of that - from atmospheric pressure (14.7 psi) resulted in 360 psi at TDC, so I think you may be out with 16:1, 15psi (gauge?) and 240 psi.
You do have to use absolute pressures, rather than gauge pressures for the formulas to work.
The SAE paper covering the 1959 air injection supercharging is 920843 (it was written about much later than 1959). I don't have a reference for the earlier "Ramaire" air ejector work.
In summary:
A S/C 2.7l six replaced 6.1l V8.
Air was stored in 2 x 2l spheres at 3000psi.
The auxiliary poppet valve used to introduce the air used a variable lift mechanism to control the amount of air introduced.
Fuel savings of 18% highway, 9% city were thought possible.
Peak savings were higher:
@ 20 mph, 47% w/o compressor operating, 35% with.
@ 60 mph, 25%, w/o compressor operating, 15% with.
There's probably a page or two around that will do the calcs for you if you plug in the relevant data.
Yep:
http://www.tribology-abc.com/abc/thermodynamics.htm