To figure out the tension on your serpentine, you find the longest spread on the belt that is free. Push/pull on it. It shouldn't deflect no more than 1/4 inch.
When you replaced the belt, did you check the play on the idler pully(s)? Did you the serpentine correctly? Smooth side run against the smooth pulleys and the teeth side run against the grooved pullys.
My YJ had the teeth side running against the smooth idler. Squeaked every start up. I fixed it....