Two different upgrades
Estus Shards increase the number of flask uses when handed to Andre. Undead Bone Shards improve healing strength when burned at the Firelink Shrine bonfire. Finding one does not replace the value of the other; keep an eye out for both as you explore.
Allocate for your playstyle
Andre can move charges between regular and Ashen Estus. A melee character usually needs little FP recovery, while a caster wants a useful balance. Begin with a conservative healing supply and adjust based on how often you actually run out of FP.
Find a healing window
Distance is not automatically safety. Heal after a committed enemy attack misses, during a long recovery, or behind reliable cover. If the boss is standing idle and watching you, a flask can invite an immediate punish.