Not to derail, but instead of saying 180 - Age + 5, isn't it just easier to say 185 - Age?
The Maffetone formula has a few adjustments; you use them as applicable to the individual. So the + 5 may or may not apply to another person.
I agree with @Kettlebelephant's post above.
@jhpowers, sounds like you're doing things just right.
in order to "own" a bell, do I need to complete the entire 5 minute swing cycle with a heart rate that follows the same formula?
IMO, which comes from Al Ciampa's teachings, no.
"owning" a bell just meant you could complete the five minutes without breaking form or falling apart in any way.
I would add (from S&S book) that you can do that strongly any day.