Best tactic for spawning a delve boss?

Hm... from PoEDB (scroll down a bit, search for Delve Biomes Boss and the graph above):

Minimum depth spawn, increasing chance of spawn up to a certain value.

Vaal Outpost - 83
Abyssal City - 150
Primeval Ruins - 250

I think that once you reached max chance you can just delve horizontally. Still, due to the area difficulty, you may choose to run at slightly lower depth for better survivability. Just do whatever hardest content you feel comfortable with.