AC guinea pig...

I do two thing to get it filled on a hot day.
1. Hose down the condenser constantly to lower the low side pressure.
When the engine is idling on a hot day, the system pressures go up, making it harder to fill.

2. Put the Freon can in a bucket of hot water to raise the pressure in the can. Don’t get the can tap wet.
When gas is flowing out of the can, the can cools, lowering the pressure.

I am too dumb to put it in as liquid, so I stay away from that.

edit: said evaporator in first post, meant condenser (the one in front or radiator).