Reverse the thick/thin arrangement and discover the error is equal and opposite. Hmmmmm. This means I need a 1.5 thickness thin washer. Go and youtube some guidance on how to actually sand a washer. Make a washer-holder from a block of hardwood with a recess cut with a 22mm spade drill. This holds the washer nicely on the flat sanding disc and on 100g wet/dry. Make myself some 0.045" washers and re-try the STBD wing. Realise that the other lobe still has thick washers rather than the plans 816L, so reverse the arrangement and discover my sanded washers are too thin. F*&k. Sand up a new pair at 0.050" and they work perfectly. One of the discarded 0.040 washers works fine on the Port wing, and everything is dead-nuts accurate. Win, win, win!!!
For the record, and for insertion into the FOH, final result for a dead-nuts wing/spar setup.
STBD upper 0.050" custom washer STBD lower 816L
Port upper 816L Port lower 0.040" custom washer
See the pix for my custom tools. The wire with a magnet on each end is invaluable for installing/removing washers inside the spar. The hand-made angles from 13mm tubing with 10mm square rod inside where required were most usefull. Either the 7mm ply discs or the tubing set up to either push a bolt-head or nut up were better than the plans 3" extensions. Bit of 5mm fencing wire was good for driving the bolts out, won't work when the FWD holes are patched or covered over...