BackdoorArts
Senior Member
Could have mentioned it in the Scott Kelby course blurb, but anyway, he mentioned an iPhone app called NDTimer ($.99) that will take your metered shutter speed (w/ proper aperture) and calculate the exposure time with an ND filter. Not only that, it will also stack filters for you. Tried it today and it works great. I waste more on bad coffee - so do you. 
Apparently it's also available for Android.
Three60 Digital Design Co.
Apparently it's also available for Android.
Three60 Digital Design Co.