I always add my extract late, like you I'll add approx 1/3 of the extract at the beginning (assuming this is an extract brew and not a partial mash), then only add the rest with about 15 min remaining. I do this for every style of beer, the extract will add a darker color to the beer if it's in the beer for the whole boil. You also get better hops utilization when adding the extract late.
I can't see any benefit to adding more water late in the boil. The bigger your boil is, the better your hops utilization will be, but I wouldn't mess with adding more water later. Evaporation is just a part of the boil, we compensate for it in full boils but you don't need to worry about it so much in the partial boils, just make sure you have an adequate amount of top up water handy.