I have been thinking about the same thing for my bike. What line do I use?
I decided it would be best to use the same profile as the bottom of the actual fender as it is now. Take a piece of thin cardboard and make a template by tracing the bottom line of your fender. Place it on the fender where you want to cut and have at it. I took your picture and performed just that in Paintshop for you just now so you would have a good idea of how it would look:
Hope you like
