Plywood Reinforcements - Wood Structures

I know I have a reference that gives guidelines on the appropriate ratio(s)
to taper plywood reinforcements (on wooden spars and the like) to avoid
stress concenrations, but I can't put my hands on it.

Can someone either point me to an on-line resource or go over the relevant
guidelines? My recollection is that you should taper doublers at an 8:1 or
better ratio.