If you follow the instructions exactly, it does not use the whole length of a 10" wedge ruler, so it is the size that it would be with a 9" ruler. The 10" wedge ruler is also cheaper than the 9" wedge ruler.
I bought mine here.
http://phillipsfiberart.danemcoweb.c...p/product/STP/
It actually calls for the 9 degree wedge ruler in the instructions.