Trust in your healer and a lack of communication (in pugs atleast) is typically a common trait with this subject. You got a Tank who takes a big whap and freaks out not noticing his healer has planned for it and is winding up a fat heal. But then another instance you'll have a healer who for whatever reasons can't cover you in time and you end up dying.

As both a solo healer and solo tank you have to ensure you're doing your best to keep the group alive while also trusting the others are on the same page despite them being random people you've never played with. This also assumes everyone is playing decently. Look we can't explain why sometimes a PLD just wants to spam Clemency at 90% HP, The Healer one button dps' during a tank buster, or why the BLM and DRG are sharing a spot on the AoE, It happens.

I've had times where I thought about Clemency,didn't, died, and then times where I did it and wasted my healer's spells. The only time this isn't a real problem is in groups with friends over discord with plenty of communication and trust. That's the only environment you can confidently have rules of engagements for your Clemency. Pugs? Just do it when you think it's needed and if the healer get's triggered then that's them.