At about room temperature (~25C)
R12 - 1.49g/cc
R134a - 1.206g/cc
HC - 0.493g/cc
I got these charge weights from
http://www.techchoiceparts.com/refrigerant-and-oil-capacities/volvo and converted oz to g based on 28.349g/oz
89 740 takes 1176.5g of R12. 0.493/1.49*1176.5 = 398g HC
Hychill manual says 360g.
91-92 240 takes 1105.5g of R12. 0.493/1.49*1105.6 = 365g HC
Hychill manual says 330g.
So Hychill says about 30% and the maths says about 33% (works out about 10% total). When you are chasing efficiency you'd be surprised how much difference that extra 77cc (for the 740) makes. 77cc fills about 2M of 3/8" tube (about 2 rows of the condenser), or 1M of 1/2" tube.
I roughly split the difference because my big charging scales are only good to about +/- 5g.
Refrigerant charge is based on volume. You want some liquid (a little) in the bottom of the condenser to allow subcooling, a solid liquid line to the orifice tube / txv and a mostly full evaporator. On a txv system you want mostly vapor existing the evaporator. On an orifice tube system you want a higher proportion of liquid bubbling out into the accumulator. Original volume/weight is calculated based on this theory.
This is also why when you convert an R12 system to R134a they say to start with about 80% of the charge by weight. R134a is lighter by volume. 1.206/1.49 = 80.9%. It's a little more complex because R134a is far worse at transferring heat than R12, so you actually want a bit more liquid in the condenser, but 80% is a good starting point.