I've tried 50-60 levels hunting before, with maybe a thouasand patrolling slaves, to kill population in provinces the enemy is about to take, and to get some benefit from them.
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Unus vocis. Unus manus. Unus Universitas. Unus Deus. Is est meus fatum praeeo pro totus populus.
Ut est meus fortuna.
If your only a minor blood nation or dabbling in it in order to extend your magic, I often blood hunt at 100% taxes, only adjusting the tax rate next turn to get rid of unrest, then bloodhunting again.
You maximise revenue this way, well still getting a steady supply of bloodslaves. The population remains very stable as well. Often I will have a lab in the province so I can research the turn I am not bloodhunting.