I never noticed an hp/hq relationship. I've even played around with the harder ones that pretty much need healing to solo with ending at like 1% or building it back up and no change. Or ones where I go over and get lazy and wait for it to leak down vs feeding down are the same. Perhaps it's just those random messages feedrate is good or whatever messages... though even when I was using lunar smarts my hq rate didn't go up either so idk.
As far as how close you have to be the break off point seems to be +1/-1 for single element harder to figure out on multiple elements but it seems like it just cares about each element not total. There is a small but noticeable change when you go 2 away. If it's something you care about HQ no reason not be within 1.
Part of the reason why HQ rate right now on synergy is anything worth synergizing is decently below the lvl cap and it seems to require less to break tiers though not really enough testing to say for certain how much or if it actually is tiered instead of straight lvls... but try doing those recipes at/below lvl and you will cry. Level plays a huge factor in hq. Though currently my rate on augments is more like 90% as long as I'm within 1
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