My (extremely sweet, ultra-capable) boyfriend is rebuilding my front porch. His goal yesterday was to pull the old boards off and put enough decking on to allow us to use the front door until he could get the rest of the decking on today. He succeeded in accomplishing what he set out to do, and the decking is laid from the steps to the door.
When he came over yesterday morning, he brought enough lumber to to lay the deck from the steps to the door. He knew it wasn't enough lumber to finish entirely, but he knew once he got the decking to the door, he could easily calculate what he needed to finish.
When you're taking measurements, making a list (and checking it twice!), you need to write it down. I, personally, write such things down on a piece of paper or make a note of it in my cellphone notepad app ... but what does a guy use? Why, a piece of scrap lumber, of course!