Depends on the wheel. A stamped wheel like a ProStar is more likely to flex than a cast or forged wheel. I don't think you need that much. The biggest problem is in the wheel bearings. SN wheel bearings have a tendancy to wobble when they get old which is going make that wheel move back and forth relative to the caliper. I would say a quarter inch and spin the wheels enough to double check it. It might be a good time to put a dial indicator on your wheel bearings to see if they have developed any wobble.
I actually replaced both front hubs less than a year ago, so they're good to go. I have a little less than a quarter inch of clearance right now. The wheels I have are forged, but they're not specifically designed for Mustangs. I have to run a 6mm spacer up front so it'll clear the caliper. I have a little less than a quarter inch of clearance and I wanted to see if I could get any closer for the sake of the thread engagement on the lugs. I was thinking of going with a 5 or 4mm spacer. I'll take a picture in the morning and show y'all what I'm dealing with already. Thanks for the input.
Might be a better idea to go with longer studs if that is all possible with your wheel. I've actually been looking into putting 3" Moroso studs in up front myself.
I'm looking around to see if I can find a longer than stock stud that will fit. I tried the Moroso studs up front a while back. You need to trim them a bit to keep the lug from bottoming out. I wasn't really satisfied with it, which is why I took the opportunity to just replace both front hubs. The spacer I'm running up front is ¼" thick, and I found some studs with the same knurl and thread size that are .230 inches longer. They have a slightly shorter shoulder height, but only by .05 inches.