Originally Posted by mikey1600

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.