Is it obligatory to make up or offer expiation for someone who used to perform istinja' (ablution after defecating or urinating) incorrectly in the past?

What is important in istinja' (washing after defecating or urinating) is to wash away the impurity until it is highly probable that it has been removed and the area is clean. Certainty is not a condition, because requiring certainty would be an undue hardship, which is negated by Islamic law.

So, if you wash the area of urination until the impurity is removed, that is sufficient for you, and there is no blame upon you.