That would vary depending on how you siphon your wort out of the pot. You might have next to no loss there, or you may have a quart or more (for a 5 gallon batch). There's a lot of variables there. Use your past brewing experience to take a rough estimate at how much liquid you leave behind when you go from the pot to the fermenter.
Since you are doing 1 gallon batch, you probably don't leave all that much behind.