The total refrigerant capacity is 26.5 oz, and it's very important to avoid overcharging. BTW, the PAG oil capacity is 6 oz.
The low-side pressure w/ compressor off doesn't matter much, and the pressure with compressor engaged should fall between 25 and 33 psi (according to Volvo) w/ ambient temp of 80F.

I've used the gauge you're talking about and it's kinda hard to differentiate the P in increments any less than 5 psi. B/C you added an entire can and b/c it doesn't cycle on and off, I'd drive it for a few days (assumming the gauge reads in that 25-33 range). Do you have a thermometer that can reach into the duct? If so, duct temp should get down to 40-50 degrees (at LEAST 30 degrees cooler than ambient air). I bought one of those $10 Target digital home thermometers (with a wire and "outside probe" temperature reading) and I feed that little wire into the duct.