Make Reuel tell the player how much HP they can still gain from this quest until it becomes daily (due to hitting the HP threshold). This can be shown to the player in the dialogue when you talk to him (prefered), when you accept the quest or when you finish it. Displaying the HP thresholds themselves will also be nice, since not everyone visits the forums to check on such details. This is a small quality of life change that will help out players estimate how many quests they will need to do overall or how many are left when almost done. Warriors and Buccaneer will especially appreciate it since they need to do much more quests to hit their HP threshold. It will also help some players understand why the quest suddenly becomes daily.
Anything to get more of this knowledge in-game instead of needing to rely on guides is welcomed. Some flavor text would indeed help explain this to the player without feeling too much like a video game. Since Reuel is a researcher, changing his opening text may make the most sense since he can explain why he can't make multiple sets of potions per day anymore. Maybe something like this: "My, $playerName, you seem to have taken a liking to my concoctions so far! But it appears that your body is adapting and my current potions are becoming less effective as a result. Thanks to your help, I've been able to devise some stronger brews that should be able to grant the same benefits as before without losing efficacy as you become used to them, but the process takes longer. Thus, I can only make them once per day. I would be grateful if you would continue to help gather materials so I can continue my research. What do you say?"
We will see what we can do to make the threshold more transparent ingame in a future patch. Thanks for your feedback!