AGFA recommendation was not to use Rodinal 1+50 for HP5+. It happens that AGFA thought 0.65 was the minimum usable C. I. and 0.61 is about all you can get out of that combination. Their number is before you even consider minimum amount of Rodinal in the tank. To each his own. Most of us consider 0.65 too high for normal subjects.
I think from what I have read if you just wave the Rodinal stock bottle around the developing tank, you will get enough fumes in it to do a roll. (If you believe that, I have a bridge in Brooklyn I'd like to sell you.)