The first URL is a good hackers guide, best I've seen.
The second more conventional. Notice the reference to
isobutane mixtures in both (sources listed in first).
http://www.vettenet.org/acfaq.txt R-12 replacement FAQ
http://www.startext.net/homes/ggoodspe/ACCONVRT.HTM R-12 to R-134a
Refrigerant Conversion