As long as the system is not left open and only opened long enough to repair a leak or replace an offending part, putting in the same amount of lost oil before evacuation should get the job done.
Two points though; with PAG there are three different viscosities, make sure you use the correct one, and replace only the amount that was lost, don't overfill.
PAG is hygroscopic as heck. Get the system repaired and put in the PAG at the last minute. Take the top off the PAG can, pour out the measured amount you need, immediately cap the can, pour in the oil and seal it up. Pumping down an extra amount of time is advisable. If you have a vac pump or recycle machine that will allow you to evacuate overnight it won't hurt.