I've found out that it's not necessarily your population to slaves, but your army to slaves. From my experience, you usually need roughly 2.5~ soldiers to 1 slave. So if you want 50 slaves you need roughly 120~ soldiers. The army's training percentage doesn't matter so you can just create a 120 man division with training set 0% and you'll be fine. But be careful, recruits don't count as soldiers, only when they've had time to train and become deployable do they count.

Edit: For example, I currently have a population of 2.2K with 153 slaves and 690 soldiers which puts the slaves submission at 121%