40 tooth rear will be very close to the 17/45 combo with 15/40 being slightly lower. That's what I run when I go strictly off road. You can always change the front to a 16 or 17 later on if you plan on just street riding. Here is a link to a sprocket calculator. just type in Hawk in the filter and then select Hawk 250.
http://woodsware.aciwebs.com/gears/ You can play around with that to figure out what you want to do. I'm currently running 16/40 because I have been doing about 50/50 on and off road cruising 55-60 mph on the highway.