Derailleur adjustment with compact crank

[lunacycles]

This is true, but it is easy to have too long of a chain on a compact if the chainstays are relatively short.

If your rear derailleur hanger is designed and aligned correctly for the gears and components you are using (e.g., "road") you shouldn't have the rear derailleur guide pulley bumping up against any cog with the b-tension screw adjusted correctly...just hasn't been my experience. Even if this were to be the case, I'd rather live with a little chain noise in the small/large combo than risk ripping my rear derailleur/derailleur hanger off due to a too short chain...

I determine chain length by "dummying it up" in the small/small combo, and ensuring the chain is just short enough that there is no chain rub against the bottom part of the rear derailleur cage in this combo. I then shift through all gears to ensure it can handle them, keeping in mind stated derailleur capacities as stated by the manufacturer. This has never failed me.

And I am trying to understand: what does chainstay length have to do with it? Thanks.