@Shane - so it's overdue, but I can report back now on the photopaper tuning method - in short, it's great! less than 1/2 hour to tune a galvo.
At the weekend, my y-axis galvo went haywire - and started gaining about 1.5/2mm per print on 90 mm parts, so that suddenly I was confronted with a 90mm part being printed at 100mm. So I dug out the photopaper and made a test pattern - it works great, only took me about 20 mins to complete tuning, and because the test pattern is 120mm across - measuring the patterns on the photopaper is good enough to tune the diode to 1/2mm over 120mm - which is about as good as I got previously when tuning the x-axis the hard way.
1) remove build platform and put empty resin tray in
2) in preform load and orient test pattern (attached) to the axis you're tuning
3) put strip of photo paper aligned with test pattern part in resin tank - cover with clear perspex to hold it flat
4) start print - wait for about 10 layers - then interrupt print by lifting cover - and measure pattern on photopaper
5) If pattern measures 120mm plus or minus 1mm - you're done - otherwise,
if pattern is too large - adjust trimpot anti-clockwise, too small, adjust trimpot clockwise
adjustments should never be more than a full 360, and when you're within 2 or 3mm - you should be making 1/4, 1/8 turn, and even finer adjustments.
6) replace paper and perspex cover, lower cover to restart print - goto step 4)
See attached pictures showing four iterations of the above instruction sequence. The first picture shows a measurement of the test pattern from the first iteration, and the second measures the 4th and final iteration.
Unfortunately - unlike my X-axis galvo, there now seems to be something very, very wrong my Y-axis galvo. It doesn't seem to be stable - after tuning it's about right for maybe 3 prints and then it starts gaining 2 or 3 percent expansion every print thereafter. So this was the second time I tuned with this method, hence why I only needed 4 iterations. I've ordered a galvo set from supplier listed on the wiki link above - at this rate it looks like I may have to try replacing the y-axis galvo.