Mick, I went through this exercise recently. My 8 inc SCT measures 29 inches around the corrector plate cell (thank goodness for dress makers tapes)
On the Dew Not site they list straps for "8 inch Telescopes" on one page, but this page gives the length of the straps.......
http://www.dew-not.com/specifications.htm
I picked the 30 inch strap and it fits with a little overlap. If in doubt go longer as you can just overlap it as much as you like. It secures anywhere by the Velcro.
Michael
