I think you can also go to Rawson's web site and download the manual.  With that you will find some formulas to incorporate the ratios Devin talked about.  That would also include seed cell counts (12 finger....30 air...etc)  which eventually works out to a seed cal value that you put in which in effect calibrates you up.  I use to do a seed drop test count (also in the book) but have found out that if you have your sprocket ratios correct then your seed cal value will be right.  A quick drop count will tell you that but then I would go ahead and use the calculated value.

We did that again this year on a fresh planter overhaul and threw in given number of bags and since we calibrated the Rawson radar factor to match real GPS speed we ran out right on the money.

Arnie