The shutter speed you need to use will vary depending on the light conditions you're currently shooting. Your goal is to use the ND filter that allows you to have good exposure at a shutter speed as close to double your recording framerate as possible.
For example, if you are shooting 24fps, you want your shutter as close to 48 (ie 50) as possible. If you're shooting 60fps, you want your shutter speed as close to 120 as possible.
If you're shutter speed is way to high for the scene, try going to the darker ND filter in your set and visa versa.
ND filters are only necessary in still photography if you're trying to force a longer shutter to blur motion.