Welcome to MeleeRumble! :-)

That's what I do. I also track gunheat to eliminate invalid waves, and account for the case of them being hit and firing at the same time (or at least, between two consecutive scans).

For that vulnerability to occur the enemy wave would have to be very low power - bullet less than 0.75 - and they'd have to wait for it before they shoot. Eventually I want to check for bullet bonus before I count a wave as hit, it would be much more complex but could lead to a significant score increase. The trouble is that I don't know if it would help much because I don't know how many waves are incorrectly labelled as 'hit', so I'm not sure if this is worth it.

I've heard that gun accuracy can go up by scanning targets just before fire time, so maybe I'll give that a shot next.