Rarely do I quote by eyeballing. I'm terrible at it. I can't tell if it's 30 ft or 100 ft. So I one of those measuring wheels that you walk behind. Walk the length, walk the width, multiply length×width×rate, and I'm done. The customer seeing me doing this knows that i am making an effort to give them an honest price, and not just making up the most i think I can get out of them.
And I usually quote to the penny.