My setup: 18X7.5, +42, 225/40-18

I've found a couple of setups with this offset running 8-9" wide wheels, but none with 7.5.

Let me see if I understand...

My rims are +42, so adding a 5mm spacer would put the offset out to +37; 10mm to +32, 15mm to +27 and 20mm to +22, correct?

I've used the calculator that I found in one of CIONIDE's posts, but I can't figure out how much extra room we have to fill up. I'm leaning towards a 20mm spacer since my wheels are a 1/2" narrower than the 8" rim recommendation that I found. I also plan on using either DF210s or S-Techs, so the rear will be 2.1-2.2" lower, another concern.

Here's what the calculator shows for +15mm spacers:
Suspension: 100% clear (leaves even more room than OEM)
Fenders: Package will stick out 28 mm (1.1'') farther.
Wheelwells: Will fill out the wheelwell by extra 3 mm. Almost no difference

Here's what the calculator shows for +20mm spacers:
Suspension:100% clear (leaves even more room than OEM)
Fenders: Package will stick out 33 mm (1.3'') farther.
Wheelwells: Will fill out the wheelwell by extra 3 mm.

Am I on the right track?

 

With your tire & wheels specs, on DF's or S-Tech's, go with 15mm rear spacers, no front spacers, and be done with it. You'll sit back from dead flush about 1/2" both front and rear (they will match for flushness instead of the rears being out more with 20mm spacers) and you shouldn't have any rubbing issues.