Yet another A/C Question

[pwogaman]

My 85 300SD A/C worked fine until the low pressure switch kicked in. I've since gotten my Section 609 certificate, some equipment, and some R-12!

My question is . . . approximately how much R-12 need be missing prior to the low pressure switch kicking in in an otherwise fully functional system? In other words - assuming everything in the system is working (notwithstanding the possibility of a small leak) - how much R-12 would I need to add to the system once the low pressure switch kicks in to get the system back up to a full charge?

I realize that there are several variables (e.g. noncondensable gases) which may influence an accurate response, but what I'm looking for is an approximation based on experience or on whatever voodoo you might rely.

I want to know if I have enough R-12 on hand just to get me through the rest of the summer until I get my recovery/recycler going (that's another story) before I start poking holes in cans - so to speak.

Your best - educated - guess???