You can save it for Cascades, which reduces AoE damage for the group, which is what we require the most. ASTs can let CU tick for more than 3s on phase 2 when the party is stacked (and WHM usually uses Cure III there). You also have to consider Time Dilation affecting Aspected Benefic ticks, which will reduce the amount of casts necessary to keep it up 100% of the time. Another thing: it's smart to line up Lightspeet with LA to get 2 more GCDs during Lightspeed; you can normalize that in your equation as well. Synastry also allows you to heal two targets at the same time, which means that some of the individual healing you added to the tank numbers is not going to exist; I don't know how to calculate that, though.

A calculation mistake on the WHM part: you considered 600 potency for Medica II (probably from copying Aspected Helios' numbers), but Medica II is 700 potency, which will make WHM more efficient.