The first link aligns with what I know about it: above 70 ng/ml is where side effects start happening. I stay below 70 with 5000, but for other people, they may need less or even more, depends entirely on where you live, and other factors that are known to burn through Vitamin D (such as being overweight or drinking caffeinated beverages or experiencing high stress).

Based research I've seen, for optimal wakefulness and cognitive ability, 50-60 is optimal, below 40 is too low, above 70 is too high.

You may have to adjust dosage over time to dial this in.