The offset is how for the mounting surface is from the center of the wheel, and the backspacing is how far the mounting surface is from the back edge of the wheel. The 5 1/2, both early and late 6, the 7 and 8x15 factory Fuchs alloys all have 112mm backspacing; only so much of the wheel (and tire) can fit to the inside of the mounting surface without hitting the springplate (and probably the top of the shock mount as well). Because this distance is constant, as the wheel gets wider- where the mounting hub is in relation to the center of the wheel changes. The 5 1/2" wheel has 42mm offset (as you already know). The 6x15's offset changes 1/4" (6mm) to 36mm as the wheel is 1/2" (13mm) wider to the outside. Similarly, the 15x7's offset is 23.3mm as the mounting surface in getting that much closer to the center of the wheel, and the 15x8's offset is 10.6mm, again with the added width to the outside of the wheel, the mounting hub in that much closer to center.
The 4 1/2x15 is a little different as it has the same offset as the 5 1/2" wheel (42mm); the added width is 1/2" on the front and 1/2" on the backside, so the mounting surface's relation to the center of the wheel doesn't change. The 911R 7x15 is different as well, as it's the same as a 6" wheel on the front with the extra inch added to the inside; this was done to get 7" wheels on the back of the stock fendered 911's of the day for competition (and are now very rare/expensive). These would fit Great on the back of a Speedster... with some low clearance springplate bolts and a little trimming on the top shock mount....there are wheel shops that can cut apart a 6" alloy and add an inch to the inside of the wheel...a couple of places around here do it...I think it's about $200/wheel...
The 16" alloys follow the same pattern, with the 6, 7 and 8" wheels all having 112mm backspacing and the same offset as the 15" wheels. There is a 16x8" alloy (951 part# instead of 911, off of the 944) with 125mm backspacing and 23.3mm offset instead of the 10.6mm of the 911 wheel, so if anyone is looking at used wheels it's something to know.
Ron, I realize you get this, but I thought some of the other guys might get something out of this (it's late, but I have to wait for the Ibuprofen to kick in before I can sleep anyway, as my knee is aching like crap). And you're right, you'll gain some clearance- 1/2", as the backspacing of the wheel is the same as the 6".