I plan my post boil volume so I'll leave enough beer behind after kegging to harvest the yeast, a good quart or two at least, but how much will be up to you. Who advises pitching new wort onto the cake? I've heard of it being done but bad idea. You'll be way overpitching, and really carrying over a lot of trub, among other things. Leave beer on the cake, swirl up and dump into a sanitized container, when ready to pitch you can pour some of the beer off, swirl, let trub settle for a minute or two if you want, and pitch as much as needed.
You can also separate some trub at harvesting by letting it settle for a few minutes before pouring off into your container. Since your yeast cake will be at least 4 times what you need to repitch, you can lose a bit at harvest and at repitching.