You have an interesting idea here, however ...

From a naming convention standpoint, you have broken pattern that could lead to conflicts with other Order Total Modules ...

You will notice how there is a pattern to the naming of the constants ...

You really want to maintain that pattern so that the module more or less "owns" the pattern ...

First you use this pattern:
MODULE_ORDER_TOTAL_QUANTITY_DISCOUNT_blah

But then you leap to:
MODULE_ORDER_TOTAL_LEVEL_blah

Then to:
MODULE_ORDER_TOTAL_DISCOUNT_blah

You may want to give that a think through and re-establish a pattern that more or less "belongs" to this Order Total Module so that there is less likelyhood of a colision in the configuration table or in the running of the module ...

Thanks again for the contrib ... looks interersting ... :)