Bubbers - 50 ft is about right. When the Nimrod was new the procedure was full power against brakes for all take offs. After about 5 years they were finding cracking around the main undercariage attachments, which were attributed to this procedure, and we switched to releasing the brakes as the engines spooled up. Field lengths were adjusted for take off calculations by 50feet.
If you were on a conveyor belt, but the brakes on the conveyor belt rollers were on, would the earth move?