Okay, residuePhysicsTime is probably what I am going to use for both areas (spawning and despawning). There will be no performance hit, and it will be consistent. I will get on that.

Also, my "fancy mathematics" was used to quantify how inconsistent the spawning is under the current method. I will still want to loop it more than once each 100ms time step (which I will call a spawn cycle) if there are entities that spawn more frequently than once every 100-200ms.