In time, you'll have a flat rate book in your head. You just won't quite realize it. It's funny how after a while I can "ballpark" something to myself in my head, figure it all up, and find that my gut feeling was darned close. I can walk around an old house that needs rewired, gauge the difficulty of the fish to each hole, and keep a running tally of each hole's "flat rate" to use as my final price. I don't suggest this for the new or the timid, but after a while, you just "know" because you've done that same task the same way a million times.
Ranch house, accessible attic, no attic floor, install paddle fan, change box to Westinghouse Fanmaster Jr. fan box, install 2x4 lumber for support, $299 every day of the week. Takes 1 hour to 1-1/2 hours on site, depending on attic clutter and furniture.