I feel like Darth has read about this, but understood it wrong.
He is correct that 1 armor is 1% hp tho, so yes at 800 health is when Armor runes are strictly better
but here is the thing, it is 800 health, but it is NOT 800 hp. What I mean is, if you have gone thru effectively 800 health, from using potions, regen or lifesteal. Then Armor runes are superior.
Honestly, Flat HP runes are pretty garbage, I personally suggest scaling HP runes over flat ones
By level 6 Scaling HP surpasses Flat HP, so as long as you don't die before level 6 Scaling HP should even beat flat armor
At level 18 its 9 Armor vs 215 hp
Oh and Scaling Armor is fine too, Again overtaking at level 6~7 and provides 3x more armor at 18
I would hardcore suggest flat armor on Junglers since their 1~7 is their most important levels, that require the most amount of survivability possible. On ADC it is SLIGHTLY less important, and it is very important to have a solid amount of survivability all game long.