Need offset help please

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I'm planning on buying a set of aftermarket wheels for my Z, but I just can't seem to get the hang of this wheel offset thing.

Have a Base with the stock 18" wheels and my understanding is that the stock offset is:
front 18x8 43mm
rear 18x9 15mm

Gonna keep the 18 wheel size but go with TSW Interlagos RF's
front 18x9.5 40mm
rear 18x10.5 27mm

I want to try to get the wheels as flush with the sides of the fenders as possible, but I don't want to have the tires really poking out too much if any at all. So given the numbers above what am I looking at to get the set up I'm looking for to get flush wheels.

Also, I'm planning on going with 245/40/18's up front and 275/40/18's in the back, and eventually will be lowering the car with a mild spring drop (1 to 1.5in) so clearance should not be an issue right? I should also still be able to get the car aligned to factory specs after the wheel mods and even after the drop too from what I've read. But before I pull the trigger on the wheels and tires I just want to make sure everything is going to work.

Thanks in advance to anyone who can help me.