Wrap card around marker or pencil for smaller tubes.
Insert in body tube and release tension, until the roll is only 2mm smaller than the tube, put clear tape around center.
For all standard 2.75" motors.
Make a mark on the roll at 2.5" from an end. Put more tape around the other end, the one that goes in the tube.
For any motor size just measure the motor and make your mark on the roll 1/4" less than the length of the motor.