The issue with a middle ring made to mount a granny to is when that ring is worn out the replacement costs more (if it is still made years later). A proper triple crank will end up costing less over the years IMO and have far easier to find replacements.

As to what type of crankset a ft der is made for- thousands of riders have done fine with their "double specific" ft ders from when that spec wasn't a reference. I think what is driving the assumption of specific double or triple ft ders is 1- the marketing of "better/faster" **** that we have gobbled up for decades, and 2- the lack of most current riders not learning how to make their bikes shift best. But this boarders on other rants Andy