It is easy to use runs saved (based on Mike's PC-to-ERA numbers) and figure how many runs created you can give away before the upgrade in defense/downgrade in offense becomes a net loss. Does that make sense? To be honest, it doesn't feel like my sharpest writing.
As for runs created, I do not know which formula HBD uses, but most formulas in use are accurate when cross schecked against actual results, which is to say when you add up a team's run created by players it often is close to the number of runs a team actually scored.