I still think the easiest way for anyone, especially newbies, is to mark your mash paddle or kettle in 1g. increments (fill it with one gallon at a time, mark, repeat).
Mash according to 1.25qts/lb or whatever ratio you choose. Figure on 0.19g/lb. absorption, so whatever's left is your first runnings. Know your preboil, if you don't, use batch size plus 1g (boiloff) plus 0.5 (trub-loss). Preboil-minus-first runnings=equals=sparge volume...then heat just a little more than this to be safe.
When sparging, shut off the valve on the mash tun when your kettle is at preboil volume. There is hardly any math involved here. As you brew, you can see if you're coming out with where you want to after the boil (how much wort goes into the fermenter). If it's low, your boiloff is higher than you thought (or you used a ton of hops). Up your preboil volume accordingly to make up for it.