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Go for 20mm all around. Should bring the wheels out close to flush with the fender.
Really, it depends on how low you're dropped. Or more accurately, how much uncorrected neg-camber you're willing to accept in the drop.
I'm planning on a set of 15mm spacers for Winter, when remounting my OEM Sport Rims. Honestly, looking at the pics in chris_s' thread, it's a convincing argument to go 20mm. But Chris has the AWD car with 7.5" sport rims all the way around, dropped on AWD Swifts. I've got the RWD with staggered sport rims, dropped on RWD Swifts.
So much inner argument about 5mm, LOL!
[November edit: I bought the 15mm set. Very content with the decision.]
it occurred to me after I got my camber fixed that guessing too high isn't just an issue of it not looking right, there's a good chance of some serious tire on bodywork contact, particularly on the front when the wheels are turned.
SO... that should help further complicate your decision and/or regrets!