Ya gotta weigh the stuff as you can't determine proper pressures merely
from gagues like the R12 days.  With the smaller amounts of refrigerant
the over/under charge by percentage is critical. The variable
compressor compensates to provide spec pressure regardless of amount,
to a point. Best to evacuate and charge with measured weighted amount.

A good shop has a machine that will pull the old stuff out, weigh it,
filter it, and then replace / replenish it for maybe 40 bucks. All
automatic and unattended. Might take 30 minutes.

Or you could buy a can and a hose & just guess. While you're there,
price a new compressor in case your guess is off the mark.