I am in the same school as Randy -- leech telltales are pretty useless unless you are a monoslug sailor.
As in airplane wings the stall on a sail comes from the back edge and goes forward. The lift is in the highest camber of the wing.
At the lift area the aft telltales should be flowing. The leech telltales should always be stalling (that is where the stall begins)
If your leech telltales flow, that means "you are loosing gobs of power," as a quote from Randy Smyth himself.
Randy and I have always concurred that you need your telltales at the high lift area of the sail. On most sails that is about 9 inches aft of the luff, assuming you are using a rotating mast. That usually puts the telltales right around the highest point of the lift of the sail.

Some folks put another set about 9 more inches back and keep them between a stall and flowing and this seems very fast. These are just to let you know where the stall coming from the leech is located and how far forward it has come.

On airplanes you have the same situation -- the back part of the wing is providing no lift, just the area where the highest curve is located.

Think about it. Why do you suppose modern sails have gone from low apect ration to tall, skinny, high-aspect ratio sails? Because the leech of the sail is not nearly as productive as the area that creates the lift. Jet wings are much more high-aspect say than a DC 3.

Some food for thought. Thanks for listening.
Rick