A/C Recharge (without knowing proper quantity)

I have done automotive A/C for some time now but have started a business doing mobile work on heavy equipment. Approximately 70% of the time there is no tag showing the proper weight of refrigerant required. About 20% of those units have a sight glass which helps resolve the problem on those, to some extent. In some cases I am doing the work for a dealer who supplies me with the information.
My problem is with about 30% of the units that I work on. I have been charging to a high side...

A/C Recharge (without knowing proper quantity)