So, after this thread I was helped to realize I was running way too high of EC for lettuce... and had some out of balance nutrients. I was using tap water with 200-250ppm and then adding nutes up to 1100 or so. oops.

So I switched out the main res with a lower nutrient solution using tap water...

And, I did a few lettuce plants with *just* RO water and general hydroponics nutrients, up to 600-700ppm. WIth no calcium or magnesium..

The image attached is a sample of what those turned out... not good... good experiment - now I know what calcium deficiency looks like.

The tip burn was much clearer an indication... the other thread picture was nutrient overload - browning/dying of the new growth, versus this was burning of the tips of old growth even.

Hope it helps someone to identify their problem later...