George, it's 5-1/2 miles from Cross Fork to Hammersley Pool. You'll climb 1000 feet in a little less than 2 miles starting shortly after leaving Cross Fork, then drop about 700 feet into the valley of Hammersley Fork. Your local REI or EMS should carry the Guide to the Susquehannock Trail System; map 5 shows this area.
If you're concerned about grades for beginners, another option may be to park off East Fork Road, just south of the village of Conrad on the Sinnemahoning Creek. It's a much gentler walk to the pool, but it's 9-1/2 miles each way.