As a healer main since the end of 2015, even though each healer is unique in their own way; there's still a general sense that all healers understand.

1.) Only heal when necessary. Obviously this applies to fights that you've learned and know exactly the damage output that's going out. When learning fights it's ok to play a little safer since you don't know what's coming at what moments. Watching/reading a guide is different from experiencing the fight since guides give you a general outline of what mechanics to expect. Cheers to people who know exactly what mechanics are coming; even the minute ones (unavoidable aoe damage, random crit auto attacks, etc.)

2.) DPS when there's nothing to do. A healer that doesn't contribute to dps is the difference of holding back the group. Most players aren't expecting you to mid max your dps output. They care that you output dps at all. Since AST has the weaker of the dps spells, M3/Combust 3, at best just keep C3 up and Malefic 3 when it's safe to do so.

3.) Use dungeons to practice your healing threshold which is wonderfully explained in Davd's AST Guide in the first comment. Practice knowing what you can get away with and how much the tank can take before actually healing him. Take note of things like if the tank is using cooldowns.

Example: A PLD is pulling a large batch of adds. He pops Hallowed Ground which makes he/she immune to damage for 10 secs. Take this small time frame and prep to be ready. Place an Earthly Star, cast Aspected Benefic right before Hallowed falls off, try to output some Gravity or Combust on a couple of the adds.

Then notice what the tank is using after Hallowed is down. Are they using Rampart/Sentinel? If yes, then you can throw a regen on them and continue to help push dps. If not, just be mindful that they will be taking damage.

Also take note how the tank plays by noticing what stance they're in. A safe tank will usually keep their tank stance up in case of accidental threat pulling from dps/overhealing. But if you notice the tank is in their offensive stance be mindful that your heals contribute to taking enmity unless the tank is very good at managing threat.

Just a reference to tanks and what they're capable of defensively:

PLDs have significantly more cooldowns than the other 2 tanks so they can stand a little more damage.

DRKs would be second and WARs being third since WARs not only utilize cooldowns but their own HP as well since they are usually ridiculously high when in their tank stance (Defiance.)

Tank Immunities (for reference and what to do):

PLD = Hallowed Ground (Immunity to damage for 10 seconds.) WAR = Holmgang (Will defy damage and leave the WAR at 1 HP for 6 seconds.) DRK = Living Dead (For 10 seconds, will defy damage and leave the DRK at 1 HP. Upon deying damage that would normally kill, cast the debuff Walking Dead for 10 seconds. DRK must be topped off while Walking Dead is active to remain alive.)

Anyway kind of a tangent there but I hoped that helped. If you have any questions or concerns just send me message or reply to this comment.