Casual player here, so forgive me.

What's the rule about when you can pre-measure and when you can't?

You're not allowed to pre-measure but players can get around this by using actions that require range (like a TL) on a target out of range, then overlapping the range ruler onto the desired target. It's especially dumb with Barrel Roll, you TL at a range 4+ ship , check if you are within range 1 of the guy in front of you, then after the TL fails you decide if you want to barrel roll forward or take a focus/evade/whatever. It's bad sportsmanship but ......

Casual player here, so forgive me.

What's the rule about when you can pre-measure and when you can't?

You're not allowed to pre-measure but players can get around this by using actions that require range (like a TL) on a target out of range, then overlapping the range ruler onto the desired target. It's especially dumb with Barrel Roll, you TL at a range 4+ ship , check if you are within range 1 of the guy in front of you, then after the TL fails you decide if you want to barrel roll forward or take a focus/evade/whatever. It's bad sportsmanship but ......

It's bad sportsmanship but people continue to complain about it despite being perfectly legal.

Alternative question.

Would the VCX-100's rear arc that can fire torpedoes override this cards primary arc requirement?

Because the special arc only mentions it's use in attacks I would guess not.

So does it work with Nera Dantels or is it still inside her firing arc?

The Sesmic is NOT an "attack" so it is unlikely to use anything that expands the reach of normal torpedoes.