Before you begin...
First, fill your pot with say, 5 gallons of water, bring it to a nice rolling boil for an hour. After an hour, cover it and once it's cool, measure what's left. The difference between what you start with and what you end up with is your boil-off volume. For the sake of argument let's say you get 1.5 gal/hr as your boil-off volume.
For a 5-gallon batch you want to END an hour's boil with 5-gallons so you should START the boil with 5-gallons + your boil-off volume in the kettle. Call this your pre-boil volume. Given the made-up assumption above that's 5 + 1.5 = 6.5 gal.
When you mash shoot for ~1.25 - 1.5 qts mash water per pound of grain. Given your ~14 lb grain bill that's 17.5 to 21 qt, or 4.375 - 5.25 gal. Remember the grain will absorb some of the water, so you won't get it all back when you're done the mash. Let's just say you go with 18 quarts or 4.5 gal, because easy numbers.
When you're done with the mash and drain into the kettle, the amount of water you need to reach your pre-boil volume is how much you need for the sparge.
So if you put 4.5 gal in the mash, and you drain to find 3.75 gallons in the kettle; and you already figured you need a 6.5 gallon pre-boil. So sparge that mash tun with 6.5 - 3.75 = 2.75 gallons of water. The grain has already absorbed all the water it can you're basically guaranteed to get out the same volume you put in. Once you get the hang of it try to get "equal runnings" which is where you get 1/2 your pre-boil from the mash and 1/2 from the sparge.
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