Change the line quickly, the least amount of time the system is open the better. If you feel the need to dry out the dryer (not a bad idea when a line has ruptured and been open) do your pull down for the leak check then turn the vacuum pump back on for several hours. Hot days are better or a heat gun on the dryer. You want all the moisture boiled out of the system.
As far as fill amounts, keep the condenser cooled with a spray nozzle on your garden hose and watch your gauges. Without cooling the condenser you'll get false readings on the high side. Assuming you have an r134 system just google the charts for the pressures to match your current weather.
I know it's not the simple answer you were looking for but it's
The way it's done when you don't have fill amounts and/or there's problems