Charge was calculated by size of receiver height and diameter. and mounting direction ( vertical or horizontal) We all have books and literature from united refrigeration and refrigeration research. That will give you the capacity of it. That is how our company and a few of the 20 plus year guys have always said to do it.
At my previous company I was always told to charge to a til glass.

I have been hesitant lately of over charging I had one case over the summer that I over charged by about 6lbs and got a call back on the WIF not holding temp. But that was charging til a clear glass and it bit me.

So now Ive been trying to use as literature as possible when charging. I also do agree with you guys that once ok but multiple times odds are that it is more than likely not a equipment issue. But everything was installed and charged to the letter so that is the part that makes it odd. and when a new headmaster is installed everything acts properly.