No, you need to multiply the R12 capacity spec by 0.75 to get the correct amount of R134a. Nonstandard components can throw off this calculation—a new filter-dryer that's a different size than the original, for example, or a parallel-flow condenser instead of the original serpentine type so the system has a prayer of doing a good job for you.
Lookit-a-thermometer is fine for a quick, general evaluation of A/C system performance, but 50° isn't some universal magic number; the temperature you want to see varies depending on ambient temperature and other variables.
You charge an A/C system by weight, not by guessing based on what a thermometer says. Also, there are no meat thermometers that read anywhere near low enough for this job.
A/C work by guess and by gosh is a pretty dependable way of breaking stuff and wasting money and ending up still sweating. It's better to do it right.