wait like even on squish?? this isn't for lategame tanks it's for adc/mage/shitoffense jungler (I build hydra lw on lee cos I'm a bad person)
Like lategame I'm going to be instagibed by anybody who wants to getting like 10 more armor isn't going to save me is it? but for trading early game flat is pretty worthit no?
I'd always go flat armor seals on ADCs, Jax, or people who would get bullied in lane really easily. As for champions who aren't expected to die in lane as much, it comes down to personal preference and who your lane opponent is.
I still use flat magic resist glyphs on one of my ADC Mastery Pages and for mid lane. You should be fine unless you want the whole buffet of options.
Honestly I do think flat HP seals are situational as heck. Most of the time the more specific options (armor) would scale better, and would be specific enough exactly to what you need in your lane.