Refrigerant charge

I have three Cavalier coke machine (pre-1960) that was charged with F12 and after some repairs (replaced some coroded refrigerant lines and added access valves and a filter/drier), I charged it with R414B. I have a low side pressure of 15-18 psi and a high side pressure of 145-150 psi. The frost line creeps down the evaporator outlet towards the compressor but does not reach it. Does anyone know how to determine whether a machine is properly charged, the data plate is no help?  Are my pressure readings right or can this be determined by the length of the frost line? Any response is appreciated.

Thanks------- Larry