Offset calculation
 

I am currently having a bit of trouble determining if these wheels will fit and look good.

Currently I am on stock base wheels with 20mm spacers all around.

Stock:
18x8 +43 front
18x9 +15mm rear

With 20mm spacers it should be:
18x8 +23 front
18x9 -5mm rear

Right?

Wheels I am looking at are:

18x8.5 +25 front
18x10 +25 rear

According to the calculator I used
Front:
Inner clearance: 8mm LESS
Outer: EXTEND an extra 4mm

Rear
Inner clearance: 43mm LESS
Outer: RETRACT by 17mm

So just guessing, I would need to leave my 20mm spacer on in the back and it would increase to extend -3mm in the rear no?

Am I looking at this all wrong?

you are looking at it right, but do the calculations before your spacer is added in, otherwise you will forget about them.

you have forgotten about your front spacer, if you have a 20mm spacer in the front already, IF you take this out the new front wheel will still extend 4mm outwards from where your front sits now currently. (calculator will say extend 24mm when given your original wheel specs to new specs without spacer). If you leave the spacer on, remember your new front wheels will effectively be a 18x8.5 +5

now for the rears, 18x9 +15 to a 18x10 +25 will give you +3mm extending outwards.

so if you like the look where your rears are now, leaving the spacer on the rear, they will still extend 3mm outwards when you put the new wheels on. If you take the spacer out, your NEW rear wheels will sit 17mm further inwards.

Makes sense. I would take the front spacer off, but leave the rears. Honestly, as it sits not, the rears are probably 5mm away from looking perfect (to me). The fronts, I don't think it would poke, as long as that is the case, I think I should be good.

...so I hope :)