Filter factors for Cokin A series

Hi everyone,

I’m wondering if you know where can I find info about the filter factor of Cokin A series filters. All I’ve done is I measured the light with a meter and then held the filter in front of it but I’m not sure if it will be accurate.
For instance red filter’s case has a coef +3 written on it but it doesn’t seem that it’s as dark that I should decrease the shutter speed by 3 stops.

http://cokin.com/en/13-filters

Chose your filter and go to product details (the former series A is today's series S).

Thanks! I really appreciate it! I’ve found what I was looking for. Except the red filter the light meter was accurate.