i replace our chains when the inside-measurement blades of a caliper set to 132.60 mm will fit between two rollers. (In practice this means to open to 130 mm, drop the blades in between two sets of outer links, and open the blades until they butt up against the rollers.). If even one of several measurements reaches 132.60, replace the chain. This is actually easier than measuring between 24 rivets with a ruler but does indeed usually equate to 1/16” over 12”, or 0.5%. Measuring a longer section requires removing the chain and making sure there is enough tension in it to get an accurate measurement. Ideally you don’t want to remove a chain until you know you are going to replace it.

Even on the tandem, our cassettes last a long long time, a chain a season and a half. We shift gears a lot and I do bias toward the big ring on the larger half of the 9-speed cassette and away from the middle ring and the smaller sprockets, for that reason.

Lube only when really necessary. On a road bike, a chain should never get so dirty that you have to remove it to remove the dirt attracted to it by over-oiling. The black stuff that builds up on chains unavoidably is steel dust — it’s magnetic, try it! — from wear. If there is grit from sand, too, you are adding abrasive material to the chain that is harder than steel.