Regarding the front of the executive desk, this is what I found on sbyrktct's website, saying NCARB suggests needing only 18":
"
Front of exec desk:- The curved front suggests that it should not directly face a wall or have furniture back up to it
- There is no evidence to suggest that the full front of the desk requires 36" clear
[ *** Note: NCARB has recently stated that they recommend providing 18" clear across the front of the exec desk. ]"
http://arehelp.webs.com/sdOf course, if your armchair is facing the desk, then you need 3' in order to provide access to the armchair.