Bobby Mcgee, From Master Airscrew props, "2-blade to 3-blade conversion: decrease the diameter by 1" and increase the pitch by 1" if possible. Adjust to 2" for larger diameters (14" and up)." SO, I guess it depends on what brand of prop's instructions you want to follow. It looks like, as with so many other things in this hobby, there is more than one "right "answer. My conclusion is, "I guess it doesn't matter that much".