If no-one comes forward with the answer or a mathematical equation...... my advice would be to buy some inexpensive dressmaking ribbon, say 1" wide, and find your domestic broom handle as a slightly reduced-scale template. You'll only need to wrap say a 1 foot length to establish the proportions. Then with a bit of fairly simple maths you should be able to scale-up the dimensions and figure the answer as to the quantity you will need for the exhaust, before you over-or under-purchase the real (expensive) stuff. As a rule of thumb though, such jobs always seem to require more than you have to hand -- think bicycle handlebar tapes for example.
