Look at the label in the car. Mine says "40oz of R12" (not exactly, but you get the idea.) Doing the 80% rule, that's 32oz of R134a.
Turns out I chickened out with 24oz in my car ... compressor was running full, air was cold, and it had about 45PSI on the low side in 100F weather (that's not surprising), 325psi high side (note: Harbor Freight gauges, I make no claim for accuracy
Two 18oz cans complete would be just about right.
But let's cover a few more things. You spend $100 on it? Even on the 3.8, that's not much on the flush / accumulator / orifice tube / PAG oil, since those 18oz cans are about $40 each around here.
You DID replace the accumulator and orifice tube, and flush the system, and use the PAG oil in it, after replacing the O-rings and lubing them with the PAG 46 oil or with Nylog Blue, right?
And then pulled a full vacuum on it, right?
If not, then you've wasted that $100 so far.