A program for calculating the strength of the guard on random maps in Heroes of Might and Magic III, called H3CalcGuards.

At the very least, the fact that such a Pandora has a value of 15000 is definitely correct.

You can check in the map description in the scenario information: if it says monsters 5, then you need to take Guard strength 1 point higher than usual when respawning (in the treasure, it's already at its maximum, which cannot be increased further, so this reinforcement of the guard doesn't affect it).