Why not just get a shop to do it? Evac the system, take the car, fix it, then bring back for recharge. That way you're not releasing what remains in your system to atmosphere, and you're sure that the air in the system is purged and properly refilled.
Too much and it won't work very well, same with not enough. It's not like old r12 systems where you just shove it in and watch the little eye