What is the perfect offset?

[Meat Head]

I'm sure JD is tired of answering all of my stupid "will this set of wheels fit with this offset?" questions! So I think if I can get some opinions on what is the best all around offset for a set of 18" wheels (7.5"-8" front and 9"-10" rear) on a 986 Boxster, you won't have to read all my redundant wheel questions . Also, would the same offsets be appropriate for 19" wheels?

One other question about offsets. If I have a rear 10" wheel with an offset of 58, and I put a 15mm spacer, it would change the offset to 43. Is this correct?

I'm currently looking at a set of 18" Carrera III wheels with front 8"/57 offset and rear 10"/58 offset. I know they would need spacers for proper fit but I just want to finally get it straight in my head how to figure the offset/spacer formula .

Thanks in advance for all of your input on this guys!