It's roughly what bormoth has said to swap a captive to a servant. The minimum subservience is 35, but it also requires loyalty 10 which (for captives/servants) is largely influenced by subservience (it can be offset by the other stats, particularly fondness or love, though).
The best way to raise overall subservience is to do both basic and advanced obedience training; it directly affects the value subservience settles at and is likely to add the obedience trait that helps further. There's also obedience reinforcement to help raise it up.
I'll think about adding a toggle to swap how the glossary is sorted; having both is probably good.