Consider waiting until flame out to add the late addition. It will be hot enough to pasteurize the extract, and you don't have to worry about bringing the wort back to a boil.
There has been a lot of talk about how the volume of the boil and the amount of extract you have in the boil can affect your hop utilization. You could possibly end up with a beer that is more bitter than intended if you only add one pound of LME to 3 gallons of water. While the best idea would be to play around with beer calculus or some other brewing software to determine the ideal early extract amount, just as a rough guess, I might try to keep the ratio of extract to water consistent with what it would be for a full boil. For instance, if this was a five gallon kit calling for seven pounds of malt extract, I would add 4.2 pounds of malt extract to the three gallons of water, and then add the remaining 2.8 pounds at flameout.