Cutting front wheel wells how much to cut?

I just pulled my motor yesterday to rebuild it so I figured this would be a good time to get rid of the front fender wells. My truck is not bagged yet but when I hit some good bumps the wheels rub some. My question is does anyone have some pics of their cut wells? I wasn't sure how much to cut out. I'm hopefully getting some 18" navigator rims so I want to make sure I have enough clearance for when I bag it. Thanks for any help in advance.