How wide is the wheel?
A -6 on a 10" wide wheel will stick out further than a -14 on an 8.5" wide wheel by 11mm (almost 1/2 an inch). Stock is -18mm x 9" wide so measure out how far you want the wheel to sit then calculate from there.
A 9" wide wheel would work. If you went with an offset between +19 and about -6 you wont need fender flares. I ran a -12 on 20x9 without flares and the tires stuck out slightly, but I liked them that way.
And yes I did mean to put +18 (it's actually +19mm, my mistake) in my prior response.