I'm building an app but I have a doubt

You def should account for that delta, and you'd behoove yourself to actually read at canopy height with your fixtures but you could use a constant per light fixture type

You can make that table with the offset that you want: https://dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/ Best is if you get a infrared thermometer and measure your leafs temperatures a bit, and then take an average offset with the difference you see in the room. My sensor is very close to the canopy, I do not use the offset.

I HAVE DOUBT YOU ARE BUILDING AN APP!!!!

google for an offset.. this is reference material and not an opinion. memories fail.

it's more than that... but if you want a more accurate offset, get a infrared thermometer, or borrow one, and compare leaf temp at various locales within the canopy that you care about and average it out... that is the offset you should use from current temp. It's 1C and probably approaching 2C... though with good circulation, it may be reduced. The rate of air being whisked away from leaf surface will help reduce offset. Also, there's no reputable data that gives rock-solid VPD targets for this specific species - and it varies by species. So whatever you are following, be willing to adjust based on whatever you can observe in the plants... test different levels... though context of genetic diversity creates a thick fog of war. Easy to attribute differences from one grow to the next simply based on what you are focused on - a very common phenomenom in most people. See enough, be as unbiased as possible, and it can lead to a better environment provided for sure down the line. Suggestions for soft-stemmed, flowering plants are a very good start. May the bias (not) be with you. - marijuana yoda The other option ... since this is local-only, just get familiar with the VPD you see. from atmosphere. it may not be exactly correct, but it will be just as correlative over time. An offset doesn't impact rate of change. You can be familiar with outcomes of the erroneously calculated VPD just as easily as the correctly calculated VPD in this context. Whether you see 1.2 or 1.35 is irrelevant, if it means the same thing to you (rough vpd difference due to typical offset) When you communicate to others, that's when it won't be as useful.. but tehy can just subtract .15-.20 for common sense ranges of temp/rh combos. i don't worry about the offset. my perceptions of the exact kPa numbers involved are the only 'false' thing... I'm systematically trying a lower VPD in flower to see how it goes, which is irrelevant to the actual offset. Using a more appropriate VPD, relative to what i see, in vege seems to have definitely improved rate of growth - but only talking about saving 2-3 days off a ~40 day vege-sized plant. Not potting up at the right time could erase those benefits. Still worth it.. i'm a big fan of efficiency. But, while it helps me make informed decisions, if i discuss what i shoot for or what i've observed won't translate as well to others, which for me is not a much of a concern. YMMV.