For ashe, each % of crit chance increases her damage by (1+crit damage)%
so with your runes + IE, each %crit increases the damage by 1.7% rather than 1%
With just the 20%crit from IE, thats 34% more damage per AA.(without your runes it would be 30%, so your runes would add 4% damage)
with Phantomdancer, you'd have 55% crit, your runes would add 11%damage. if you can get +7.66% critchance with the same runes, critchance would give you more dps at all stages, else it would still be more powerfull with a rushed IE and befor getting any crit item.
In Generall, ashe scales with crit and crit damage exactly the same way as any other champ, just that she uses the statistical average damage per AA rather than useing RNG.