Lantus Correct Dose

Probably questions for pros, but my thoughts :
Basal is a bit high if it's dropping 40/50 while asleep for 6 hours, though only ever so slightly. The target should be flat.
If your BGs are generally in control, 7 might well be the right amount.
Putting one more unit on will mean you'll drop further at night - I'd not recommend that.
Removing one will be a 14% change. Removing 0.5 if possible (half unit pen) might be worth a try. If not possible, try removing one for a few nights, see where BGs go. It's difficult to guess what the effect will be - experiment is about the only way.

"someone must take 8 unit of lantus and it's the exact daily needed of basal" is an interesting statement. I'm not sure it's possible to say something is the exact required dose. It might well be the recommended starting point, but all recommendations are estimates and if you're looking after your sugars and testing enough, you'll have the feedback necessary to tweak them a bit.

As @evilclive states:-

In an ideal situation, your basal insulin would give you a flat(ish) line if you didn't eat and your activity levels were steady.

Basal testing will tell you if your dose is correct.