Let me try again.....Why try to predict all of this using all the complicated science and formulas, consulting the Myan Calender (adjusted for the Moon phase), gravity calculations, and throwing in the current price of eggs in China while burning the feathers of a black chicken and rattling cat bones? Get a piece of wood - mash paddle or something - and hold it upright in your boil kettle. Pour in a known amount of water, say 1/2 gallon, and take your trusty Barlow and carve a notch where the wet and dry meets. Pour in another known amount, say 1/2 gallon, and repete the notch.As you boil away, occasionally put the stick in and see where you are at. Cut the flame down and let it settle a few seconds and take a measurement. Not there yet? fire up the flame. At your desired volume? Start cooling; open another beer. This takes away every variable, temperature, humidity, altitude, etc....100% reproducible volume.I know, expansion due to heat...bla, bla, bla.Enjoy the brewing experience.
Well, if you don't consult the Mayan calendar, then you might ending up dramatically changing your hop utilization as you try to fiddle with the boil volume. But instead of doing a kabuki dance, why not plan to overshoot your boil-off target and then top up if needed? That's what I do, but I generally have a good feel for what the boil volume looks like and I fiddle with the burner (though never my boil time) and I usually hit my volumes dead nuts. 5 gal exactly yesterday (to the accuracy of my measuring spoon).
there is the amount the volume decreases as a result of cooling