To add to what With the Birds said. Since the current Phantoms are a fixed focal length, the only way to control exposure is with the shutter speed or ISO. The generally accepted convention for video shootings is to use a shutter speed that is twice the frame rate. This provides natural, expected motion blur that everyone expects to see. Using a high shutter speed appears unnatural, and harsh. Ok back to the ND Filters. The chart you provided lists the amount of stops each filter provides. If you have a shutter speed of 500 and you want a shutter speed of 250 you would use a filter that provides 1 stop. Most video in the US is shot @ 30FPS so we would like a shutter speed around 1/60 of a second. I believe that DJI GO just calls this 60. So, until you get a feel for the current conditions, you would take an average measurement without a filter and note the Shutter speed (keeping the ISO @ 100) For example if the shutter speed is 500 you would need 3 stops. 500,-250 is 1 stop, 250-125 is 1 stop 125 -62.5 is one stop. you would use an ND8. Also remember to shoot in Manual mode. I hope this helps.
