I've pressed the skins fairly well, what ever amount of juice that I get, I use that amount divided in half as the amount of water I'll use for the second run.
Let me explain, I got 6 gallons of juice from the pressed grapes, I added that back to the original fermenter. In a separate fermenter, I take 3 gallons of water (acidulated at about 30 grams per gallon of tartaric acid) and bring the SG up to 1.090 with sugar, I take all of the skins and add them to the fermenter and punch down several times a day, the yeast that is still present on the skins will start fermenting again.
I've got 2 real nice batches of second run wine, both are stronger than a rose', real nice early drinkers.