By my calculations, the 13-lb mash would soak up about 1.3 gallons of water (0.1 gal/lb), and your efficiency would be maximized by going with a water to grain ratio of 1.5 qts/lb, so that you would need 4.9 gallons to strike, and then approximately 3.3 gallons to sparge. Every system is a little different, but experience says for me and my system that this would put me within 1 quart of my desired final volume.
I also try to monitor volumes in the kettle towards the end of the boil as well. If your kettle is pre-marked with volumes quart-by-quart as mine is, you can do this too. Occasionally I boil longer or shorter to try to adjust volume directly in the kettle.
Don't forget to account for all your water losses, which include liquid lost in the mash tun, the hot and cold break, and trub and samples in the fermenters. All those things can add up to a good 2 to 3 quarts. So if you want to end up with 5 gallons of beer, after the boil you really need somewhere around 5.5 gallons. My calculations above have already taken these losses into account, at least for MY system.
Good luck. Let us know how it turns out for you.