Not sure what .9% stretch means. The usual way to determine when to replace a chain is to measure 12 links with the chain under slight tension. It should be 12 inches exactly. If longer by 1/16 inch, replace it. But that works out to .52%.
A "stretch" of .9% would be .108 inch, a hell-of-a-lot for 1500 miles. BTW, I get about 5,000 miles out of an Ultegra chain (riding on the road in mostly dry weather, lubing every 100 miles).