You said that it is at the limit of its adjustment. I think you misunderstand how the tensioner marks are used. They are used when tensioning the belt and then do not change until you loosen the bolts again.

When replacing the belt and/or tensioner, you should always do two things; follow the instructions to get it tensioned correctly and spin all pulleys and accessories to check for roughness before putting back together.

BTW, my original tensioner lasted about 200,000 miles.

Good luck,