I use the same kettle and spoon every time. And if Sam Smith made a whatever style I am curious about I am am gonna try it.
When ever I know exactly what volume is in my kettle, I make a mark on my spoon.
I got, minimum, marks every two quarts out to four gallons. And one quart marks everywhere near three gallons.
If I got 3.0-3.5 gallons of fresh boiled wort in my kettle and 8-10# ice in primary ( a pint is a pound the world around) I am going to have pitchable temps more or less instantly and I can dilute with room temp water to my desired volume.
If you are going to be with the same kettle and spoon for a while, mark your spoon. If not, cut notches in a stick.
I try to keep my volume "constant", +/- one quart during the boil. My hops balance comes in real close to expected, and my stove does a lot better dealing with small volume additions.
So I know my start volume, and I a run a countdown timer. My notes might say "one quart water added (45)" and "one quart water added (30)" and so on or whatever. When I am brewing I strive for "the zone". I try to do the mathletics outside the zone.
Cutting all your hops back 10% tonight will probably work fine,