Cadillac Repair: lbs of r12 in 93 cadillac deville, 134a freon, refrigerant pressure

Question
how many pound of R12 freon is used in a 1993 cadillac deville  I'm converting the R12 in to 134a and i need to only use 80% oh what r12 was, a guy told me to take this calculation : (lbs of r12)times .8 = X divided by 16= how many cans to use,  is this a good calculation?

Answer
The system holds 2.42 lbs of R-12.

2.42 lbs x 80% = 1.93 lbs of R-134a

NOTE: you will most likely run into a problem with the sensors as they are NOT calibrated for R-134a freon and the CHECK ENGINE light will come ON and or you will get a LOW REFRIGERANT PRESSURE warning on your display.