Corner Panel Resonator Formula? Theory?

Hello, I am currently trying to improve the Sub Bass in a Post Production Studio. It is already well treated for above around 200Hz, Ceiling Cloud and all, and sounds decent to around 120Hz. The dimensions are approximately 18' * 10.5' * 11.5'. I determined that a Panel Resonator in each corner would probably be the best bet. I thought I should first attack around 63Hz as that frequency seems to sound the worst. But, although I have found the Flat Panel Resonator formula many times, I have found nothing significant about Corner Panel Resonators. All I have found says to double the distance from the corner to the panel for the chosen frequency. This averages to that frequency. With the waves bouncing on the sides of the trap before hitting the Resonator Panel again, it would seem that the distance travelled would lengthen and the distance to the corner should then be shorter. Maybe? Is there any more theory available on this? Have I missed some information somewhere? Any links, websites? thanks, Martin.