#53 is interesting. My wife really likes it. If the F intertwined with the DNA coil was less "swooshy" I'd probably like it better. Thanks for your efforts!
My wife is surely to like one of your designs the best. I'm partial to #69. You even make a case for the epsilon instead of the element of symbol. To me, the letterform looks more like an LEX as opposed to an LEF. I have an idea that might not work, but I'll see if I can convey it. If you "doubled the frequency" of the helix such that behind the epsilon there was a helix intersection, instead of being within the middle of the helix, the slope of the helix might be more vertical allowing the line integral to look more like an F than an X. Thanks for the nice work!
it has been a long time since I took those classes in college at UT! - so I hope I understood what you wanted. Happy to adjust any way you prefer... I like your wife's taste! Honestly hope you get a logo you love and represents your objectives.