Its not a certain percentage either. Its +/-15mm to stock overall diameter. This is for NSW btw.
Given urs is a 03-04, stock tires are 205/55/16? and u want to compare that to 225/35/18.
Using http://www.1010tires.com/'s tyre calculator this is the result I got for you.
Stock tires = 631.69mm overall diameter
New tires = 614.68mm overall diameter
631.69mm-614.68mm = 17.01mm!
Looks like you are illegal by 2.01mm
Note, that the width of your rim would also affect the overall diameter but it was not an option at that site. However, the difference is minimal.
As Crapdaz has mentioned, 40 profile would be the most suitable. Not 35.

