The proper R134 charge is the R-12 charge times .9 then subtract .25 lbs. For intance a 2.50 lb R-12 charge would require: 2.5 x 0.9 = 2.25 then 2.25 - 0.25 = 2.00lbs R134. Conversion can be done successfully. All of the R-12 will need to be properly recovered. The compressor should be removed and drained of as much oil as possible. The accumulator will need to be replaced with one with an R134 compatible dessicant. You will need to buy a conversion kit. Remove the schader valves from the R-12 fittings and screw on the R134 fittings. Add the proper amount of PAG or universal compressor oil to the compressor and system. If the existing rubber hoses are not torn do not replace them because the R12 oil helps seal the pores in the hoses.