Questions about knog. Blinder Road R70

hi sven http://b2b.knog.com.au/media/wysiwyg/files/product_instructions/Blinder_Road_Rear_R70_quickstart_guide.pdf There are three different straps (rubbers). The largest is for a diameter of 32mm+. I would measure your aero seat post with a string to determine the circumference. 32mm diameter times pi =3.14 (geometry was a long time ago :) = around 100.5 mm circumference think 2-3 cm more could also work.