Essentially, the link asks for a minimum and maximum lat-lon for the grid you want to calculate the magnetic declination for. According to the help page:
The grid calculator allows for calculation of magnetic field values for an area or a point. In order to calculate a grid of components for an area, you fill out min and max longitude and min and max latitude values in order to define a bounding box for the area of interest.
To input an area:
Max longitude = 85 1 39.6192 W Min longitude = 125 1 39.6192 W Lon Step Size = 1.0 Max latitude = 60 3 3.7836 N Min latitude = 20 3 3.7836 N Lat Step Size = 1.0
I am not sure what the correct "bounding box" for the entire United States is. I used an open-source tool called Bounding Box Tool, but it didn't help. Someone posted on Quora and said the "extent" of the bounding box was:
Extent: (-124.848974, 24.396308) - (-66.885444, 49.384358)
But... I'm not sure what that means.
Can someone help?