I think the highest efficiency comes with sending arround 35% of your thieves or it might be just what worked for me on occasion where I tried to ns, sending more will kill more, but if you fail the loses will also be higher, so not really worth the risk at a certain point.

The wiki states that Nightstrike Kills up to a maximum of 13% enemy soldiers at a rate of 0.67 soldiers per thief.

And that other troops die at a much lower rate. I'd estimate the % for other troops to be somwhat below 2% for troops at home and less still if away, but it is just my eyeballing and more likely than not some might actually have the real numbers, I've mostly been on the recieving ends as I normally play attacker, and I haven't asociated with top kingdoms who probably got a few people who could give the exact math amongst them.

I believe that optimal range will also affect those numbers, so if your nw is much higher or much lower effect will be lower.

Running no elites and getting my military ns'ed for most of last war I lost arround 1/3 or was it 2/3 def specs to ns while the off specs lost feelt rather insignificant compared to what was lost on attacks, perhaps 1/3 or 1/4 the rate of troops at home.

NS of course should not be the only way to deal with an opponent, you ns so that you can break a specific target, or so that you can break that target twice with all of your core players or so that you scare them into training defense only, you create targets or distract the opponents from what they really should be doing in a war situation.