The typical rule of thumb for conversions is 80% of what the r12 capacity was, ie. 3 lbs of r12, you would charge 2.4 lbs of 134a.
Also, r134a has a tendancy to seep through r12 hoses over time. You may want to consider having some new ones compatible with r134a made up. I'd also change the drier and drain the oil from the compressor and add synthetic PAG oil which is what the r134a systems like. Doing anything other than the above may result in some issues down the road.
