Dr. X Online de'Lioncourt's Holy Water Calculator for Ravenblack's Vampires! A Dark Alleyway
So, if the formula is NBP = OBP - (OBP^0.6)
Then simply enter their current bp into the box below and hit calculate to see what a single hw will do to them.

= - ( ^ 0.6)
Blood loss of:

So, that's the simple one. Now, you can choose to calculate a certain number of hitters (or number of hits, at least) on the bp vampire above, or take them down to a specific bp level.
Note: You will need to fill in the box above, but there is no need to hit Calculate! above.
Number of hits: Down to bp: