When do Nightshields kick in?

I'm a newcomer to 6th straying out of long time retirement (back in the days of 4th), and I'm not 110% on when Nightshields kick in...

The crucial line is "This adjustment may put the vehicle out of range from an enemy shooting attack, in which case the shooting automatically misses."

My assumption is that if I'm 24" away from a Boltgun that wishes to fire at my Nightshielded Ravager, that Boltgun is effectively an 18" range weapon and so can't legally shoot the Ravager....is this correct? Or is my opponent's shooting attack wasted for declaring the shots against the Ravager? Basically: when am I supposed to tell my opponent that her gun's range is 6" less than it ought to be?

In your example, you are correct in saying he is out of range. However, because of pre-measurement now, the shots he declared are not wasted. He can measure, see that he is out of range, measure distance to something else, and then fire. If he declares he is in range and in fact forgets about Nightshields, if you just remind him, he will be able to select a different target.

Depending on the circumstance, he may want to change the movement of the squad/ vehicle that wants to fire at you because they forgot about the -6'' to their range. Then it would be up to you about allowing them to re-move their unit to allow it to shoot at something else.

SCP has it right, the wording about wasting the attack is a holdover from 5th edition, which is when our codex was first published. I suppose that if he pre-measures and then declares the attack that, yes, the attack would be wasted if he was out of range - officially under the rules you have no obligation to tell him about the night shields after initial disclosure of army info presuming initial disclosure is even a thing.

Pre-measuring sucks the biggest one. It's me and my friends' least favourite bit of spoonfeeding from 6th, and we always house-rule it out. If you're just playing with your mates, let them see the light and do that.
But yeah if they measure 24" between you and their bolter, and then say they're taking the shot because they forgot about your nightshields, it's their problem not yours!
The tricky part is telling them that without sounding like a complete tool. Make 'em see the funny side.

Pre measuring is the best part of 6th!!! Makes things much more tactical IMO and helps balance random charges... It's fantastic for de vehicles with night shields as you can sit on the line of fire accurately

Oh it definitely makes it easier to win, but it's the biggest fourth-wall breaker in gaming (except for knocking a model over with a die)

Takes away some of the fun for me, I liked the old way.