I rarely give thought to Aetherflow charges and this guide helps. Here are some of my thoughts.

While Sacred Soil is likely the least MP efficient use of a charge and Lustrate the most efficient, I think there is a scaling of utility/peace of mind vs pure MP/healing efficiency.

For example, the 10% mitigation of Sacred Soil may help to put the other healer and the tanks at ease, by cutting back on severe damage.

Along this scale, Energy Drain's typical 250-300 mp return is slightly more efficient, but only really gives the SCH peace of mind by keeping MP high.

At the other end, Lustrate is the most MP efficient, but in order to maximize its efficiency, needs to be used when the tank is below 2/3 to 3/4 of their max HP (or more, if we're wary of Fairy and secondary healer overlapping cures). This definitely places a little more tension when we have to let the tank drop low to maximize our Aetherflow charges.

This can really apply to healing in general, of course. For example, regen works best when everyone is not topped off, but seeing the party's HP bars below max produces just a little more anxiety.