Well I have only found Flexwatt in 3, 4, 11 and 17 inches. And I've read a lot of people recommend about 1/3rd of the tubs length. Thats what we do anyway and it seems to work out well. Gives a nice hot spot, then a good transition to the cool side. So if you're thinking on the 8-9 inch side, you should probably just go with the 11inch. Unless you can find flexwatt in a different size, then you need to let me know where you get that hehe Good luck