Porsche 911 Air Conditioning Revival. Going from R12 to R134a

Like a lot of Porsche cars out there, this 911 Carrera had the air conditioning taken off at some time in its life. When the air last ran, it was on R12 refrigerant, which is banned since 1994.
To get this air conditioning working again, all the hoses, evaporator, and compressor were changed out for new parts.

The molecules of R134a are smaller than R12 and could leak out through the hoses and flare fittings that R12 systems used. Also, R134a systems operate at higher discharge-side pressures than R12, which can cause compressor seals to leak. You need to run less R134a than R12.
I got this formula from Techtown.com. (Kurt tells me this is still too much gas and to use the pressure reading on the air conditioning machine)

Formula: (R12 Charge Specification x 0.9) – 0.25 lbs. = R134a Charge Level

As an example, let's say the R12 charge specification is listed at 2 lbs. You would do the following:

Example: (2 lbs. x 0.9) – 0.25 lbs. = 1.55 lbs.