Does anyone measure the amount of far-red light in their grow relative to the amount of plant stretching it causes?

No, life is too short..........

bruce bugbee has some videos and research related to this... it's important to know just because you can prove a statistically significant correlation doesn't mean it's an overtly profound effect... just that the relationship seems to exist and fairly consistent reaction to the stimuli. in the end, lanky plants will be lanky. squat plant will be squat... within those reams (and more between) it can have a measurable effect. blue promotes axillary growth. red promotes growth at the tip... he's got research showing PAR is not fully inclusive of all the important wavelengths, like far red, or whatever the proper terminology is called. He suggests a different range of wavelengths wider that the range of PAR... give it some spiffy name i can't recall off the top of my head. "extra" PAR lol.

I’m 100 % sure no one here owns the proper sensor to monitor far red light 😀

I have known for a while that that IR diodes in grow light cause stem elongation, but I could never find any actual numbers. I just watched another great video by Dr. Bruce Bugbee (Director of the Crop Physiology Laboratory at Utah State University) and he said:
    “We get asked a lot about how these photobiological ratios apply to cannabis. The good news is that my laboratory has a state license to study cannabis. So we've applied these ratios to predict cannabis growth, and they have the same value in cannabis as they do in all other crop plants. Adding far-red is both good and bad and that's why the sensors are so important. You want to add the right amount at the right time to get full advantage of it. More far-red can make cannabis plants taller. That’s usually bad. But it also helps expand leaves. That's good, and there's evidence it can help with slightly earlier flowering, which is another benefit. So this is emerging research.”

 Predicting Stem Elongation and Leaf Expansion: Percent far-red is a better predictor than PPE https://youtu.be/I_KPcg5eSV8?t=1111
 
    The chart that appears at 15:05 (https://youtu.be/I_KPcg5eSV8?t=905) in the video appears to say that stem elongation begins increasing around 2% Far-Red Percentage. Y-axis is delta percentage (amount of change) and x-axis is the Far-Red Percentage (far red / (PPFD + far-red))

I now own an Apogee ePar meter that can measure 400-750nm. The Epistar IR LEDs that I am looking at burn at 730nm, so the ePar meter should be able to measure 730nm. I am going to order IR only light bars and want to know how much far-red input I need without causing stem elongation issues.