That is a LOT of refrigerant! I've installed lots of A/C systems, doubt if I've ever used more than 3 cans on an empty system, and that with a rear-trunk-mounted evaporator system and miles of hose.
although not a perfect approach, I've had 100% success with just adding refrigerant until I get around 180-200PSI on the high side of the compressor. I DO hope you are using a gauge set to add the refrigerant (about $50 on sale at Harbor Freight, and a great piece of equipment to have on hand).
To make a successful conversion:
1) attach a vacuum pump to the system (engine off) and run it about an hour or so to remove any system-killing moisture
2) To test the integrity of the system, close of the pump line, but leave your gauges attached overnight. If you still have the same vacuum reading next morning, you're reasonably assured of no leaks... otherwise, you'll need to add an UV dye to the refrigerant to determine where the leak is.
3) Start the car up, block your accelerator pedal to get about 1500 RPM's or so, turn on the A/C, and start interjecting the refrigerant into the LOW PRESSURE side of the compressor (NEVER open the valve on the gauge to the high pressure side when the engine is running) Be sure to add some R-134-compatible PAG oil to lubricate the compressor. This is most easily done by just buying a can of R-134 with oil included, available at most parts stores. A single can w/lubricant should be sufficient.
4) Every 30-45 seconds or so while adding refrigerant, you can momentarily turn the can upside down then right side up again to expedite the transfer. Be patient, it can easily take 5 minutes or more to empty a single can of refrigerant
5) continue this procedure until the high pressure side reads 180-200 PSI as stated earlier (different systems may require different pressures, but you'll be in the operational ball park at these readings). And, NO, a just a bit more refrigerant/pressure for the wife and kiddees is not necessarily a good thing. As soon as you're getting some decent cold air out of the system, with pressures somewhere in the stated range, you should be good to go.
A few other notes:
*Adding refrigerant is most effectively done with ambient temperatures in the 80 degree or higher range
* Best to set up an external floor fan blowing on the radiator while charging the system.
*Before turning on the valve into your A/C system, be sure to purge your gauge lines of air (and moisture).. just crack the connector on the hose right at the compressor connections and open the valve to your refrigerant can until you see/feel a bit of refrigerant coming out. For this reason, the best gauge sets are those which have shut off valves right at the compressor connection to allow you to purge.
After all that, I should say that some 10 years ago I bought one of those quicky R12/R-134 conversion kits at the auto store. You know, a couple of adapter fittings, 2 cans of R-134, and a hose and valve to fill. My old Chevy truck A/C had long since quit working... I didn't evacuate the system, or anything. Just slapped those adapter fittings on there, dumped in the new R-134 cans, and I swear that truck is still blowing cold air 10 years later! Point is, these systems are pretty simple and goof-proof, but as long as you're doing it, no reason not to take a few extra precautions to save you grief down the road!