I have a new Park Tools CC-2 Chain Checker. It tells me my chain is worn to 0.75%, with a few strands at 1%, and <1% across the split link.
So I have taken off my old chain. Compared to the new chain, the stretch is half a link in 111 links. Which equates to 0.45% stretch compared to the new chain.
But the new chain also has some "slack". So the old chain is less stretched even than 0.45%. I measured the old chain against a 48 inch ruler. The stretch is 1/8 inch over 40 inches. Which equates to 0.31% stretch.
So I've bought a tool, and a chain, and spent time changing the chain, for the sake of, say, 0.35% stretch, when the Park Tool gauge reports 0.75% or more stretch.