Many trailers have the holes placed where you will expect to transfer about 400-500 pounds per hole. Some trailers have the holes much closer together, and you will be transferring weight at around 250 pounds per hole.
If you need to add more weight to the trailer tandems and less on the drive axles, you want to slide the trailer tandems Forward. (toward the tractor)
If you need to take weight off of the trailer tandems and/or put more weight on the drive axles, you want to slide the trailer tandems Backward. (toward the rear of the trailer)