As we know, every 3 points in Fire Blast gives Death Sentry one more shot. So, 18 was an optimal FB level to get the max amount of shots out of DS.
I've done a little messing around in the cow level and have discovered that it is only a waste of points!
I do not mean the synergy doesn't work, I am stating that it is highly unnecessary.
When casting DS, I would always have them near me since I can swap to it while keeping DTalon held down and resume kicking. Since the DS were near, they would shoot lightning and occasionally, they'd blow something up, but weren't reliable.
When I'd leave a mob, but it was close, the DS would remain and not shoot lightning at the near mob. However, they would always (yes, 100%) use CE.
I'd noticed it before, but had never paid it any attention. Then, I actually tested it by casting DS only at the edge of the screen. DS blew up a corpse every single time.
Not only is DS too far to want to shoot, and therefore waits, it will save all 5 shots until a corpse appears, and not shoot another until another corpse is present.
The exact range I'm uncertain of, but when cast at the edge or just a bit past the screen, it will only use CE. When used for that instead of damage, we can easily save 17 skill points by casting DS in a certain range and not have to worry about wasted lightning bolts.
So save your points and lay DS in a certain way.
Also, I'm not sure if it was already a widely known thing or if it was already in a guide. I'd heard nothing about it before and had always heard of wasted shots and DS only doing lightning, so there's your solution folks!
Interesting observation. I had a play with it last night in the Pits and, although I haven't tested very scientifically, I tend to agree. Cast the trap on top of a monster and you get more Lightning. Cast it far away, more DS.
I normally play K/T the same way as you, so this is a bit of a pain though.
I noticed this long ago (and always lay my DS a bit behind my LS's), but I still max FB, since I actually use FB to take out fire immunes. If that's not that you are aiming for, I guess you can indeed not max FB.
I normally play pure trapsins (with max FB), but I have noticed that they rarely fire off all their shots. I remember reading somewhere that DS will shoot CE as far as 2 screens away if the monster is in your field of view. In the same place it said that DS will shoot at the nearest corpse(w/CE) or living creature(w/lightning). That seems consistent with my experience.