There's a common misconception that you must use 9 of a specific rune. You could always use a mix!
My tank page consists of 30 Armor / 17 Magic Resist all flat, simply because I prefer having a really strong early game which is where runes really shine. Though I suppose scaling glyphs will influence late game more than flat. Keep in mind, Nasus has a pretty beefy base and growing HP stat AND a health bonus ultimate on top of that, so defense will benefit more than health from runes. Specially since you need to be constantly in and out of the fray to last hit.
Quints/Seals are Armor
Glyphs are Magic Resist
Marks are a mix of Armor and Magic Resist
If you are shooting for cooldown to get to 40% though, chances are you will include a Frozen Heart in the mix as it provides hefty armor. In this case I would throw in full Magic Resist Marks and possibly even Quints.
Your best bet is to make various tank pages revolving around the defenses I mentioned. If you are playing ranked, you can determine what will be the best route depending on what the enemy team is going. There's not much sense to stack huge armor if your opponents are Malzahar, Anivia, Annie, Vlad, Veigar.
I love my movement speed tho so I won't be sacrificing my movement speed quints, but I'll get full magic resist marks then. Why does magic resist marks work well with frozen heart? Is it cuz frozen heart only gives magci resist and no armor?
And ya i guesss ince I have over 4k health, magic resist might be better. thnx