Normally a freon top off is done by running the ac system and watching the
"window".  At idle the window should show steady stream of refrigerant but
not fill up the window cavity.  At 2500 rpm the window should be full with a
fair amount of bubbles in the refrigerant.  Do not charge until the window
is clear of bubbles, trust me, there's a reason I know.  For a properly
charged and operating ac system the temperature drop from outside ambient to
"cold" air side is 18 to 22 deg f.  This is for a system not on recirc.
Resist the temptation to just charge the hell out of it.  It is better to be
undercharged than overcharged.  An overcharge of refrigerant will cause
damage to the compressor and other components.  Remember to purge the fill
lines prior to hooking up the fill rig and gauges.  If you are starting from
scratch with an evacuated system, hook up the fill rig and start the ac
system.  Top off the refrigerant as if it was low.  As for the amount of
refrigerant in the system, I would imagine it to be less than 36 ounces.
Sorry I don't have the exact figure off the top of my head.  I'll see if the
figure is in the book.

dave