No. I said, "the relevant ways."

On plays close to the Line to gain, anywhere on the field, the primary focus of a clock operator is on the Line Judge, who should be in the optimum position (On the line, facing the stakes) to judge if the LTG has been made, by signalling to stop the clock.

We agree completely. It's the L's job to stop the clock on a first down. As the R, the only signal I give is the first down signal, followed by the wind (if we're going to wind the clock). Our instructions are to look at the L for stopping the clock on a close LTG situation and then look at the R to start the clock.

On our crew, if the L tells me to look at the ball, we're having a measurement 99.9% of the time. Otherwise, he wouldn't be telling me to look. If the clock gets stopped to look, we're measuring 100% of the time.

This is my point. On each of these occasions there is a time lag between the official deciding to stop the clock, and actually signaling it. There is also a time lag from the time the operator sees the signal and actually stops it. Thus, at any other point in the game, there may be a second or two between the runners foot touching out of bounds and the clock getting stopped. This lag is a result of human reaction time. In the play in question, no consideration was made for the time lag.

Isn't it made up for by a similar lag in starting the clock?

Absolutely!!....Unless you eliminate it like they did in this scenario.

And that goes back to my question. For this play, does the rule say the clock stops when the ball carrier's foot steps OOB, or when the official on the field signals?

The rules do not specify to the level you are seeking, however there is an otherwise inherent understanding expressed regarding similar issues. As we are all advised, the sounding of a whistle merely signifies and announces a decision a covering official has made, so any unexpected delay in sounding the whistle does not alter the judgment already made requiring the whistle to be sounded.

It seems, somewhat obvious, that the same principle would apply to your question. The precise instant when the game time stops, is actually when the covering official decides that some action, specified in the rules, required the clock to stop. The signalling of that decision merely is announcing the decision that has already been made.

An example might be, an official deciding he observes that a runner has stepped on a sideline, and deciding that action requires the clock to be stopped is unexpectedly knocked down prior to being able to signal the clock stoppage. The time actually stopped when the official decided the action he observed rquires stopping the clock.

Officials don't usually stop clocks, with some exceptions, the clocks are stopped as a result of specific actions defined by rule.