the Vilna Gaon, the bayis shel yad may rest anywhere on the
biceps muscle. Consult your rabbi. The bayis shel yad must not
rest below the lower point of the biceps muscle.
Verify that the bayis is not positioned too low by locating the base of the muscle with one's finger and making sure that the front edge of the base is above it. If one can place the pinky, ring, and middle fingers between the cleft of the arm and the front edge of the base while the arm is outstretched, then it is most likely in the proper position according to the opinion that the base of the muscle is determined when the arm is outstretched and relaxed. If one can place the index and middle finger between the cleft of the arm and front edge of the base while the arm is bent at approximately 90° then it is most likely in the proper position according to the opinion that the base of the muscle is determined when the arm is bent at approximately 90°.
Feel along the hairline with the length of the index finger or with the tips of his fingers to verify that the bayis is resting above it.
The back edge of the bayis can reach until the top of the head, which is equivalent to the end of the soft spot on a baby's head.
Identify this point by running the tip of a finger straight up from the front of the ear.
The majority of the knot must rest on the center of the base of the skull. The remainder of the knot may rest just below it in the indentation on the nape.
Verify that the majority of the knot is resting on the base of the skull by pressing on the upper part of the knot. If one feels the small, round, protruding bone when pressing, then the knot is in position. If by pressing on the lower part of the knot one feels that it's in the indentation of the neck, then that verifies that it's centered.