I used to teach the pilots to say, "One potato, two potato, three potato, four", as in the children's chant. When you reach "four", you should be at ten degrees pitch and lifting off. The rotation continues airborne up to an attitude which will cause the plane to climb at between V2 and V2+10, usually about 15 degrees pitch up. read more
On large aircraft the procedure is exact. Once the takeoff weight and center-of-gravity of the airplane are calculated, either a computer, index cards, or a set of tables are used to find the relevant speeds. read more