casting mechanics question

[subteraneanbird]

To put it simply, casting is split into two actions:

Step 1): You hit the button. You go into casting stance, meaning movement or a proportional amount of damage will cancel your cast. Nothing else happens during step 1 Cast time for the spell is calculated at this time, based on your current Spell Speed.
Step 2): The cast finishes (usually at 80% full due to how the game handles pinging for spells). All actual effects of the spell are calculated at this time. For healing, this includes the amount healed (affected both on caster side (Divine Seal) and target side (Defiance, Convalescence). MP cost is calculated (usually value listed, but 0 if Freecure is active at this time, 1/2 if Overcure is active at this time). Secondary effects apply (the hot from Medica II, the shield from Adlo/Succor). For HOT/DOT effects, FFXIV uses a "snapshot" system where the potency of the effect is determined solely when the initial effect lands. Like the Divine Seal+Regen example from above, or like Aero+Cleric Stance.