Block the condenser airflow to get the head up to where you would be on a warm summer day. the 50 ambient would be easy to account for, But it obviously must be cool inside with a low return temp and that would be the challenge IMHO.
I agree! Also if it is a larger split with a large charge, Why make the customer pay for all that nonsense of recovering and weighing? Charge it in, You should know how to if you are in thie field, hopefully. Most units have a charging chart you can get through the manuf. you need to get your return wetbulb etc. Or tell them you will be back in the spring, they are most likely not using it in 50 degress, And if they are it most likely has a low ambient kit.
You can block airflow on condenser and use superheat calc to determine correct superheat unless its txv the do same thing only charge to subcool. Its in the book. Also if you have a heat pump to keep from lowering temp in space too much just energize 1 stage of srip heat to keep from dropping too low for charging. then switch to heat and determine if your spilts are acceptable for heating.(indoor and outdoor)
I have a switch set up that I temporarily wire into the condenser fan motor, and manually modulate the head pressure. unless there is a gas furnace to run to warm up the inside, then the cold inside temps still make it a pain to dial in. I always schedule to return when it's warm to double check the charge.
I had to do a start up today on a a split system with a gas furnace. It was 57 degrees out, but I was fortunate enough to be able to start the heat up first and let the heat run for a while when I was evacuating, so I brought the inside temp up to 84. That made it easier when charging.