VPD vs leafe VPD

leaf vpd is what matters, yes. though you'll probably see a pretty consistent offset that you can apply to the atmospheric VPD. So if it's a consistent -2C lower than the air, you can expect that to continue, though do measure the leaf if trying to be super precise. i've seen multiple ranges of suggestions for seedlings, vege and flower... so which is correct? your guess is as good as mine unless properly educated. 1.1 vpd for vege phase should be fine. some even show you can go a bit higher. These are rough estimates of what i've seen listed as target VPDs... .8-.9 early on .9-1.2 vege 1.2-1.5 flower... google all you want onthis stuff. you'll find one person says this, another says that.. i'll admit i have no idea what "ideal" is. I just do my best not to let it go too high in flower when all my lights are 100% and adding a bit more heat than before. devil's advocate -- are those sites with varying suggestions educated enough to know they are supposed to use leaf VPD and not atmospheric VPD? lol.. i'd wager this might be why there are fairly large variances between the suggestions from different sources. It's tough to know what info is not misinterpreted facts/science. A little bit of knowledge (incomplete knowledge) is a dangerous thing. A slightly off VPD is not the end of the world. It may impact the concentration at which you feed, because it impacts rate of transpiration.