The Calumet Exposure Calculator sounds like a $7.99 version of the Quick Disk, which available for free at:

Another approach is to put a ruler in the frame at the subject position, compare the field of view to the width of the frame to determine the magnification, then use an exposure table to determine the amount of compensation for the magnification factor. I keep such a table taped to the back of each of my LF cameras for this purpose. If all goes well, this table will be attached to this post as an MS Word doc file. The print is small so that it can be taped to the groundglass frame.