Hey there, I am using the same nutrient blend (MHPgardener)😉

For my indoor lettuce garden I was trying to keep it around 650ppm as well and on occasion will have tip burn as well but could not find a relation to high or low PPM. I tried from 500 to 900 range over the years and did/didn't have tip burn at all levels so I decided to blame the indoor A/C environment.

My favorite lettuce varieties are Buttercrunch, Green Ice, Red Oak, and Sparks Romaine - all from Johnny's Seeds. They seemed to grow the biggest and fastest in my systems. For me Buttercrunch and Romaine got the tip burn most often, the others did not.