That first chart has been called into question before - since it was put together by a forum member somewhere who may have done it in a hurry or smoked one too many that night. That one and the second one seem way oversimplified, but at best I guess these charts are rough guides.
The last one would be more accurate and I think comes from more 'official' sources - even though it makes things look more vague and less black and white than the first two.
Here I'll make it look more black and white.
In my experience 6.2 is too high to feed at in soilless. At least consistently.
There's no 'one' magic ph spot where everything clicks, and so there does need to be a range for the different elements to uptake properly.
But that doesn't mean we need to feed low one day and high the next- though that strategy can be useful once in a while too. .
Since in my experience the ph seems to always rise in the pot, the best way (IMO) to achieve that ph range is to feed low.
If I consistently feed at 6.2 I'm already off the scale for some nutrients and heading upwards.
If I feed low- I'm in a better position.
I generally feed around 5.6.
5.8 is considered that best possible spot where most elements are available. But that doesn't mean it's perfect.
I've feed lower and once ran one plant at 5.2. It was very happy. At the same time in that test i ran another clone of it at 6.2 and that one was very sad. To the point where I eventually threw it out since the 5.2 one was twice as big and four times as happy.
I never feed above 5.8 anymore. Possibly the occasional 5.9.
Anytime I get above 6 my plants get unhappy
