'82 911 SC muffler back-pressure
 

I plan to order a mb911 muffler from Ben.

From reading Wayne Dempsey's engine rebuilding book I understand that I should be concerned about having sufficent back-pressure to optimise the horsepower gained from SSI, etc.

Does anyone know how I can measure the need for backpressure so I can communicate those readings to Ben?

Most any muffer will do the job, even if it had only one baffle. The most common issue is too much backpressure. If you were running open headers, no muffler as in a race app, that may come into play.