My method for determining/doing flares.......this may seem a bit involved, but here it is.
Get one each tire/wheel combo of the size you intend to run.(or may run in the future)
Set the camber you intend to run.
Set the ride height.
Have at it with hammers & stuff
Cycle the suspension with tire & wheel attached to max compression.....& lock if in front.
Add spacers to clear inner fenders. (your negative camber increases as the suspension compresses).
Have at with hammers for more clearance
Then smooth & add bondo......unless you're John Kelly
This is not a job you wanna do twice....trust me.
A fine point:
Wide wheels are actually easier to get clearance as a wide tire doesn't bulge inward/outward past the wheel....What you see with the wheel is what you get so you can use just the wheel for checking as you beat the crap outta your fender. You also get a nice verticle tire side wall.