The number of threads showing isn't the whole picture - it's just easy to remember.
Depending on the design, the ball may have considerable stem length before the thread starts (as yours shows). Here, the shortest practical length (leaving some wiggle room for wear) is the height of 1-2 threads (not actual threads) below the rocker, if that lets the cup clear the body. If you have 3/8-24, 2 threads is about .083", much closer to the rocker than you show - which raises the ratio.
Note: it's also helpful to have enough thread showing to release the pushrod, which means (roughly) the ball radius.

I would guess he did the geo and saw that a shim was a needed correction. If yes, it's still needed (although I'd prefer a shim under the pedestal).