You can mash at the described 1.25 to 1.5 qts per pound of grain. There are some differences in outcome from the low to the high but negligible. What you really need to know is your boil off rate. It will be a little different for every different equipment set up.
My 10 gallon pot and SP10 burner boils off just about 2 gallons per hour. Some get as little as 3/4 gallons per hour. I ignore all the calculations for how much to sparge with and just measure the wort collected. I made a dip stick by pouring in one gallon at a time. For example, if I get 2.5 gallons from the mash and I know that I need just over 7 gallons to end up with just over 5 in the fermenter and 5 in bottles or the keg I then sparge with about 1/2 of what I need for preboil. So maybe 2.5 gallons which gets me to 5 gallons. I measure this and do a second sparge to be fairly accurate, So I need another 2.1 - 2.2 gallons for the last sparge and then the boil. This leave very little water (wort) in the mash tun which gives an accurate pre-boil, and a lighter grain bed to carry to the compost pile.