RE: 2 blade same as 3 blade?
The rule of thumb is way too simple. Truth is, it's basically BS when the 3-blades we have available are factored into the equation.
The blades on the 3's are way different than on the 2s, even when comparing MA 2s to MA3s. The blade area is the big difference. It appears that MA might have tried to match total blade area. That pretty much throws out the comparison by efficiency and any other that might have been the basis of the rule of thumb. Matter of fact, MA's suggested engine to prop chart isn't even close to what my models/engines have told me they want.
What engine/plane are you propping?