Yeah I've tapped a mirror at like 35km/h.. guy tooted (he was at a standstill) stopped and looked back, wasn't smashed or anything and was only an old VN commodore so kept going.
You get better and better at guaging the width, and once it gets narrow just slow it down a lot / walk it through almost. It'll take you a month or so before you even become comfortable about lane splitting big gaps but eventually you just try and find your way through, i even go down turning lanes sometimes and then just wind out in front of the straight traffic sitting at the lights.
oh and if you get stuck half way down splitting because the gap is too small at the mirrors etc. just stop inbetween the cars roll back a bit let them go and continue on your way.