It will really depend on which brakes you have, and whether you have a Super, or standard Beetle. Even then, results may vary depending on the car's history (was it wrecked at some point or not).
The purple 1302 in my avatar has 17x7.5 wheels bolted to 944 hubs and spindles. I *think* they're ET55, maybe ET52, I dunno. I got 'em second hand and I don't believe the backspacing spec isn't cast into the wheels anywhere that I can remember. They're aftermarket Cup3 wheels and are right at the edge of the stock fenders.
I'm sure this info gives you the utmost certianty in finding the info you're looking for.
It's been so long since I'd read what nomenclature is on the 1302's wheels that I can't remember what's on them.
*EDIT*
Ok, I just broke bad with a flashlight and a mirror, The fronts are indeed ET52. I believe you're ET40's would sit farther in...