Just to eliminate any possible confusion...

Flemish is how a string is made--has nothing to do with the material. You can make a flemish string most any material (the only exception I know of is Angel Majesty string material), as you can an endless.

With that said, you want a flemish string made from a polyester ("dacron") material--that narrows it down to either B-55 or B-50 (I prefer B-55).

I'm not real familiar with the Robertson bows, but my best guess would be 56" is a good starting point. Polyester is going to stretch/creep a good bit. Probably want it around 56.5" once it's shot in.

Chad