PSA: Crit Damage marks are better on Ashe
tl;dr crit damage runes about as good as AD early (worse for poke & 2xAA trades, better for all-ins), they break even for 2xAA or W-AA trades when you get a pickaxe, and they're a helluva lot better late.
#Early Game
Early game tl;dr There are only 3 situations where
Math
edit: I forgot ad from quints and dblade in my original calculations. (6.75 + 7 = 13.75) * 1.1 * .2003 = +3.03 damage from runes + Dblade when you crit. So I'm just adding that on to the end of my calculations. That's what that number is, where you see it.
Ashe is guaranteed to crit an extra 10% damage on targets who are slowed. She has 51 base AD.
9 AD marks * .95 per mark => 8.55 AD -> 8.55 damage (first AA) crt marks => 0 dmg (first AA) [One AA trade] AD wins by 8.55 damage
8.551.1 = 9.40 damage (each additional AA) 9 crit dmg marks * 2.23 per mark => 20.07% crit dmg .20071.1*51= 11.26 + 3.03 = 14.29 damage per crit. 14.29 - 9.41 = 4.88 damage 8.55 - 4.88 = 3.67 damage [2 AA trade] AD wins by 3.67 damage. [3 AA trade] Crit wins by 1.11 damage.
If you add in a W (100% total AD ratio) before the AA, it becomes: [1 AA trade] AD wins by 3.67 damage. [2 AD trade] Crit wins by 1.11 damage
#Later
You break even when you buy a pickaxe.
Math
1.1 * .2003 = .22 bonus damage per AD, per AA. 8.55 / .22 = 38.81 AD makes you break even on crit vs AD marks. That means 77.62 is what you need to break even on a 2-aa trade. You start off with 51 + 6.75 + 7 AD = 54.75 AD. So you need ~23 AD to break even on a 2xAA trade.
I'm done for now. If you want to figure out how much extra damage you'll be doing at like, one item, you can do the math yourself :P