VPD

What is directly around the leaves is what matters.. stomata are mostly on the underside. Leaf temps will be cooler than ambient air temps due to evaporation that occurs. believe it's a 3-5F offset, you'd need one of those infrared thermometers and gun the leaves with it. It'll be a relatively consistent offset to use if temps and rh are consistent. If the rh probe isn't intercanopy, then expect rh to b a bit higher too. good circulation should mitigate that to some extent. here's the key.. take the measurements the same way each time. This consistency gives integrity to the numbers you become familiar with... it may be an offset from reality, but it will be a consistent offset. In the end you still have to observe and react to plant to dial in whatever is "optimal" and there are limits to the human eye in this regard, especially with a handful of plants. So striving for some specific value that may or may not apply to your genetics or other variables is sort of a blind endeavour anyway. There's just not enough resesarch in this regard, yet. some good info out there to start wtih, but the plant in front of you dictates in the end. So, you can probably guess if your calculation or on high or low side given the above.. a 1.7 kPa might be closer to 1.4-1.5 kPa relative to leaf temp and local leaf RH%. But, that offset will be consistent once you learn it through observing plant and finding some point where it seems to be the best growth. it's a tool to use to help interpret what you see in the plant.. any symptoms or growth patterns etc.. the plant's behaviour is always the primary rail.