When I do it for a wee heavy I just eyeball it until it looks about like LME. That's at ~ the 1 qt left stage. I was blown away the first time I tried it at how good the beer was. Can't duplicate it any other way.
A quick way to duplicate it is to start with actual LME and bring it to your desired caramelization temperature. There is no boiling that way and the LME will easily heat to the desired temp. You have to heat it very gently or you will burn it. Heating it in an oven should work best.
BTW Martin, when I said "can't duplicate it" I meant by using crystal malts (and other malts) to simulate that flavor. I did that prior to reading about reducing the gallon of wort. Now it's just Golden Promise and an oz or two of roasted barley (ala Skotrat).