I have a small leak in my evaporator. So, here comes the cooling
season again, and until I have the money to get the system repaired, I
need to add a charge. I've got the gauges hooked up, and measured the
ambient temperature at 82 deg F.

I remember that I used to just subtract x deg. from the ambient, and
add refrigerant until I reached that value on the low-side gauge.
However, I don't remember what that value is. IIRC, it was 35 or 40,
but the synapses don't fire like they used to.

Can anyone help me out here? I fail to understand the
superheat/subcool stuff, but the rule-of-thumb has served me well in
the past.

Joe