Depends on temperature. This is about optimal transpiration which requires you to look at both temperature and relative humidity. A VPD chart can help. It won't be differenet for autoflowers and photoperiods. There's quite a bit of discrepancies as to what VPD you are suggested to should run, but it's better than doing so blindly or just focusing on one or the other (temp/rh). When looking at vpd charts, know that the leaf temp is often -5F from ambient due to evaporations occurring (an endothermic reaction that absorbs heat from environment). This in turn impacts how much the plant drinks per day, which would impact how much fertilzier concnetration you want in your substrate. Many moving parts.. no simple answer.
Basic: Simply avoid high-risk of mold and other pathogens. 40-65% range is generally safe at typical operating temperatures. A vpd chart will often color code what is a risk for molds and such, too. So you can reference this with a table of data.
more complicated.. there are suggestions for VPD at each stage of life cycle. Some say X, some say Y, so it's pretty hard to determine which source is correct and which is just making shit up.. maybe the ones recommending higher VPD weren't taking temperature measurements at the leaf and neglected to use the -5F offset, etc...
.8 kPa for seedlings -- going off the top of my head, so definitely look up some vpd tables and various suggestions.
1 kPa for vege ... sometimes see this recommended anywhere from .8-1.2 kPa
1.2+ kPa for bloom... sometimes see this recommended up to 1.5 kPa
Again, do a google search and find some recommendations and decipher which ones you want to follow... be willing to adjust as you observe the plant.
i don't have lab-precise control over my environment. So, i just try to stick near 1kPa in vege and often up near 1.2 kPa in late vege with mature plants. the resulting temperature i reference with a vpd table to set the RH% of my humidifier relative to that temperature. My flower phase is anaywhere from 1.3-1.6kPa. No idea what is optimal and others don't know either based on the range of suggestions. But, it's a good ballpark to play in -- observe and adjust based on what you observe.. it's a good starting point.
(**** my measurements are based on temp and rh readings just below the canopy, which means my actualy VPD is probably a bit lower due to the air immediatly around a leaf's stoma being cooler due to evaporation occuring ... this is not the most precise way to do it, but as long as i'm consistent, i can still observe and react in a precise way to what i see... it just won't translate as well to other gardens if not measured in same way)
why give him a simplified, reductive answer with numerous holes in it? I hate that when people give me some watered-down explanation that doesn't explain the causality of what is occurring. Just as you don't like the full answer, some people won't like the simplified spoon fed option. So, maybe don't make it about yourself and your preferences. It's good to get the full explanation and a simplified one... let them choose. If this illicits anger or frustration from you, that's a you-problem.
Seen a shit ton of bad answers this week... not just based on opinion, but objectively wrong information - sometimes simply from not reading the question and then answers a phantom question, lol. I feel sorry for the growers asking questions, lately.