Charging the AC, how do you know the quantity of freon being added?

This is just an aftermath, but I'm puzzled by what I was told. Had this guy come over for AC check/tune up on a rather chilly day, he measured the pressure of my ancient Rheem system outside, and it was showing around 40-45psi, and he told me the normal should be 65psi. He did admit that the low temperature (it was 50 F that day) will influence the reading as well. He told me he would drop by later and refill it for the cost of material, $65/lbs, and he said I probably would need 2 lbs. He came by a couple days ago, when temperature was 72 F. The reading actually climbed up to 60 psi, so he refilled it while watching the meter and made sure it was stable at 65-66 psi. Whole process was less than 10 min, and he said it was probably 1 lbs. I did ask why it was not proportional, compensating 20 psi calls for 2 lbs according to him last time, but this was only for 5 psi. But the fact that he kept his word to drop by, not charging another round of labor and seemed to know what he's doing, made me OK with the 1lbs price. But I'm puzzled, how would you know the quantity of freon that's added? The meter he used only showed pressure, it was all up to him to tell me how much I need. Thanks a lot.