Dishing flip flop or spacers behind sprockets?

I
I find it tricky to measure chainline (just how do you decide on the centre of the hub to say 0.5mm accuracy?).

You don't find the centre, you take the whole width and halve it then use it with other measured dimensions to work out where your chainline should be / is. Then you can measure from the rear dropout or locknuts to the sprocket.

Vernier Caliper, ruler, knowing the dimensions of the hub you have from manufacturers info and working it out with the rear droput spacing all help.

http://www.sheldonbrown.com/chainline.html