Yes, sight glass method on an R12 system is perfectly acceptable. When topping up my 300E with R12 every year (it needs a can every year) I don't even use the gauges anymore, just hook up and fill until the sight glass is *almost* clear.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280