Originally Posted by FBinNY

+1, any 12 inch rule (doesn't have to be fancy or precise) will be an accurate chain wear checking gauge. However the replacement guidelines apply only to chains used with derailleurs or spring tension arms. On SS or IGH bikes with a simple chain loop, the chain doesn't have any spring loaded slack and therefore cannot skip until the sprocket teeth are tiny nubs. Feel free to run your chain until it's stretched about 3%, which will take quite a long time.

To prevent excess wear, be sure the lower loop has enough slack to sag slightly compared to a straight edge laced above it.