Hi 2CV. I agree that this is the general idea but will only work if you can get the front wheel dead straight. Using the string method takes into account the different tyre sizes when you ensure that there is the same gap between the rear of the front tyre and the string on both sides. If the offset is wrong in the rear then the rear wheel will still align with the front but be misaligned with reference to the frame and the front wheel wont be straight (if that makes sense).
Re the spacers, there are only 2 removable ones and they don't fit anywhere else. The only other possible place to have one which would keep the wheel away from the RHS swing arm would be within the cush drive housing against the small hollow spindle but that would not move the centre of the wheel to the left, it would only push the RHS swing arm out.the only way to move the centre of the wheel to the left in this case would be to machine 10mm of the long spacer, i would then have to find a way to add 10mm to the sprocket side. I think an incorrect offset is more likely than a spacer that is too long. Anyway im off to the bike shop tomorrow but can feel the tug on my wallet already