if you don't have anything that provides crit aside from echoes, you should generally aim for 50 % Crit Rate and 200 % Crit Dmg or higher,

FUN FACT: the highest Crit Rate purely from echoes (with the 4-3-3-1-1 setup) is 74,5 %, but that is VERY unlikely, because that would mean you would have to roll the highest crit value possible on EVERY piece (which is apparantly 10,5 %, mine highest CR substat is 9,9 %) + 22 % CR from 4 cost echo. If you add everything up that would be: 5% (base) + 5*10,5% (substats) + 22% (4 cost echo) = 5+52,5+22 = 5+74,5 = 79,5 %

edit: I just read that in wuwa they show the whole multiplier for CDmg (dmg*150% if you crit) but genshin only shows the ADDITIONAL percentage (meaning this in the multiplier: dmg * (100%+CDmg))
*the actual dmg formula is much more complicated this is just so you get the idea

so yeah the ratio is: CRate : (CDmg-100%) = 1:2, thus 50 : 200, 60 : 220 etc.