wrote

It is calculated as weight divided by the square of the span.
Which, if other posters are correct, is not the normal way to
calculate it.

Read it right off of the FAA site, here.

http://www.airweb.faa.gov/Regulatory...dvisoryCircula
r.nsf/0/eae91a15c5e11823862569b600563fbf/$FILE/AC21-17-2a.pdf

It was on page 5 of 7, in my reader.

This is criteria for type certification, but it is the standard that
homebuilts must follow to be counted as a motor glider.
--
Jim in NC