I need to machine a spur gear with 106 teeth. I'm not having a problem calculating diametral pitch, addendum, etc. nor figuring what pitch cutter to use. My problem is with the indexing calculations. I'm using B & S plates with 40:1 ratio. Per the machinists handbook, 106 target divisions require 1 full turn plus 7 holes on 39-hole circle + 29 holes on 41- hole circle. I know 40:1 means that 1 full rev of the indexing head = 9 degs and no probem calculating up to 40 divisions. The number of divisions over 40 requiring partial turns is confusing me. Could someone break it down step by step?

Thanks!