How is a meatball calibrated and checked?
There is a vast amount of printed information available on the IFLOLS. A thorough understanding of the following list of publications is a must for an LSO:


Technical Manual, Improved Fresnel Lens Optical Landing System Mk 13 Mod 0, Installation, Service, Operation, and Maintenance Instruction (NAVAIR 51-40ABA-21).


— NATOPS Landing Signal Officer Manual (2004)

If you want a thorough how-to, I believe NAVAIR 51-40ABA-21 is the way to go if it is publicly available.

Stabilization

4.3 IFLOLS STABLIZATION INPUTS
IFLOLS receives ship’s pitch and roll information from either the ship’s gyros or SPN-46. IFLOLS receives ship’s heave information from either an IFLOLS generated ship’s heave signal or SPN-46. The IFLOLS signal is generated using the IFLOLS unit 5 accelerometer. The IFLOLS can use either ship’s pitch and roll gyro source with either heave source. When aircraft are landing using ACLS, any mode, IFLOLS should use the same stabilization inputs as SPN-46. Typically SPN-46, SPN-41, and IFLOLS will all use the SPN-46 gyro for pitch, roll, and heave information when aircraft are landing using ACLS.

SPN-42/46 is a carrier-landing radar.
From Wikipedia—

[The] shipboard units are mechanically moved (the &quot;roll angle&quot;) to adjust the touchdown point of each aircraft. With this adjustment, the tailhook touchdown point can be precisely targeted based on the tailhook-to-pilot's eye distance for each aircraft type.

Calibration
(YouTube) Calibration c. 1968.
Manually calibrating the IFLOLS from the stern of the ship from specified distances and heights hasn't changed much from the earlier days.
(navy.mil) ATLANTIC OCEAN (May 13, 2013) Electrician's Mate 3rd Class Martin Torres, from Long Beach, Calif., calibrates an Improvised Fresnel Lens Optical Landing System (IFLOLS) aboard the aircraft carrier USS George H.W. Bush (CVN 77). George H.W. Bush is conducting training operations in the Atlantic Ocean. (U.S. Navy photo by Mass Communication Specialist 3rd Class Kevin J. Steinberg/Released)
(navy.mil) CORONADO, Ca. (Nov. 16, 2016) Sailors assigned to the aircraft carrier USS Nimitz (CVN 68) take measurments while conducting maintenance on the ship's Fresnel Lens Optical Landing System on the flight deck. Nimitz is preparing to get underway to conduct routine operations in preparation for an upcoming 2017 deployment. (U.S. Navy photo by Seaman Cole Schroeder/Released)