I just found that simmilar older Scotsman CM855 have these data for charge (of R502):

Low pressure just before harvest 28 psig (-19.4°C).
High pressure 240 psig (42,4°C).
Superheat of TXV is 3-4K.

It looks like that I have been close in my first attempt to charge it correctly without knowing what is factory weight charge. Sadly, I did not measure the weight.