As far as I know, the save file only has a boolean value to tell whether items are hardened. This implies that it doesn't keep track of the strength of the Harden Item potion that was used. But I haven't looked at it in a while, so I might be mistaken.

You could test it with an editor: Use a potion of strength 1, and a potion of strength 200, and make the same exact explosion. If you get different results, try both several times to see if it's randomized.