Based on expectations of incoming damage.

Benefit of having 4 80s and all the roles covered is I know what kind of pressure my tank and DPS are likely to be under, and I heal for that. That isn’t some percentage, it’s situational.
- I do like to have them all topped off in any low pressure moment. I want people saving mitigation cooldowns for high pressure moments or tankbusters, so I make sure I heal them to where, if I was playing that role on my character in my character’s current hear for it, I wouldn’t be wondering where my healz was at...

If you let them slide to the point where they’re stressing, that’s the point where they start blowing up rotations... and then things become a mess. I only let that happen if I myself have hit that point for some other reason...

Caveat is that healer is my secondary role, I maim as a tank and DPS is probably third. But I try to regularly rotate between them all to keep my perspective current.