Do notice that every healer has in a sense a natural level of overhealing. Generally speaking, a WHM or a Diurnal AST is expected to overheal more than a SCH, for instance, as DoTs usually cause overhealing, whereas shields help reducing it. Ofc there's also the fact that SCH usually DPS, but my point is still valid to some extent. If you have access to a parser, the most important statistics you should look at as a healer is OverHealPct in my opinion. Assuming you're keeping the party alive, the lowest that % is, the better.
Take a look at this, by clicking on the hyperlinks you'll find all the data you need the have an idea of how each healer job should do in that regard.