In order to get 5.25 gallons of finished beer in the fermenter, I start with about 9 gallons of water!
For example, say there is a 10 pound grainbill.
I'd use 15 quarts of water to mash with. (3.75 gallons).
The grain will absorb about 1.25 gallons of water- that means you'd get out about 2.5 gallons of wort.
I lose nearly two gallons to boil off and trub in my system, so I start with 7 gallons in the boil kettle. I need to sparge with 4.5 gallons of water to get that, if I get out 2.5 gallons from the mash.
That right there is using 8.25 gallons of water. I "lose" a little bit in the plumbing/false bottom, so having 9 gallons ready is insurance that I don't undersparge.
For larger grainbills, you'd use even a bit more water since the grain absorbs about .125 gallons per pound.
For your next batch, keep track of the volumes you have and if you have a boil off of more than a gallon or so, start with more wort in the boil kettle. It'll work much better if you measure your volumes.