[MMcG]

Okay say a particular hardtail frame has a 68 degree head tube angle (and let's assume that is with a 5" fork) wouldn't using a 4" fork steepen the angle a degree or so?

And wouldn't running a longer travel fork - say a dual crown, slacken the head tube angle by an degree or two?

Or do I have it backwards?

 

[Repack]

You are correct. But on some DC forks, the lower crown is thinner than if it were a single crown. I think that e13 says that their reducers steepen the HA by .5deg, just to give you a basic idea of ride height to HA relations.