Yes, it is the Potion!

Found at the start, - In the very 1st map section, even.
They'll surely help you out. - Well, they're used for healing.
Found at the end, - They can be bought even from endgame shops.
They're worse than useless. - But they don't heal a whole lot at the end.
Used correctly, lucky you'll be, - Use one at 7577 health for all lucky 7's.
Used incorrectly, dead as the dirt. - Well, using a potion when low on health instead of a hi or x potion, not a good idea.
Never underestimated,
Never overestimated,
The game will tell you the exact answer. - They will ALWAYS heal 200, just as the game says.

You're up, HC.