Chain finally replaced when the rear axle reached the end of the dropouts with the 20t sprocket. It had lasted 4463 miles. For comparision, average life of the previous 7 chains, using the same criterion, was 2849 miles & maximum was 3492 miles.
It's an encouraging result, but I don't consider it a valid comparison. After 1770 miles I swapped from the 20t cog to the 22t, due to the effects of atrial fibrillation. Although I was riding over the same terrain, albeit more slowly, chain tension will have been 10% lower, & chain flexing about 7% lower. I'm no tribologist, so wouldn't try to guess the effect on wear rate.
Nevertheless it's encouraging, so I'm trying the same lubrication regime with the new chain, apart from applying lub. when the chain first starts to get noisy. Hopefully the operation to fix AF (6 weeks ago) will soon let me go back to the 20t cog.
Meanwhile, I measured the distance I had moved the axle back. 6.6 mm is about 1% chain elongation. My previous measurement techniques, showing about 0.5%, were badly wrong
. At least I now know why I've had tooth wear problems on the Galaxy cassettes & chainrings.
Edited to correct percentages elongation