Thanks for the edit - glad you were paying attention - I’ll fix the original post too.
Well I WAS being pedantic in a very obtuse way, I didn’t want the size of the flute to get in the way of a good story. 
My brain was just ahead of my fingers as I typed! 
Circumference is indeed a linear equation, however as bit diameter increases the flute size changes (dis)proportionally, which makes for a not quite so precise sum.
Yes boys and girls, never let anyone tell you that size doesn’t matter.