If you have a critrate of 100%, every hit will be a critical hit. If your crit dmg% is zero, it will just be a regular hit, but coloured yellow. If you have a 50% crit dmg, then the critical hit will be 50% stronger than a regular hit (but if you have 100% crate, there are no regular hits left, only crits or potentially weaks). Hence, if you built a total atk double compared to your base atk, that's equivalent to critdmg 100%. So build them in same ratio.
(except proc'ed hits where the skills explicitely says it can't crit - pay attention if your build evolves around this by any chance, then the base skill will be more influential).
HOWEVER, buffs behave differently to this:
- 50% ATK buff is a 50% increase to total dmg, nomatter the kind of hit (only for ATK based champs, duh)
- 30% CritDMG buff is a 30% increase of total dmg (not just an increase of the multiplier), IF is a crit hit (mind the above nuance, also regarding weak hits)
From this, just visiting the very basic notions, keeping in touch with the affinity weakness will potentially be one of your biggest dmg increases possible.
Finally, worth exploring weakness and def down/ignore def% (both increase total dmg and are complementary to each other), them might be easier to come by than this spectacular piece of gear giving exactly this and that substat.