I've been under the impression that buffs of the same type are added while different ones are multiplied. So it'd be (base) * (atk buffs) * (1-turn atk buffs) * (damage buffs) * (weakness) * (crits) when those last two can stack. The +10% crit would be added to the normal 50% damage bonus.

1.6(with crit up) divided by 1.5(without) is 1.06 repeating, or a 6% increase relative to no trainability
1.9 divided by 1.8(with your +30%) is 1.05 repeating

The higher your crit damage modifier is without the trainability, the smaller the relative improvement it provides. There's also all the non-crits where it provides no benefit whatsoever.