Stopping down when zooming 28-70 with WACP

I tried using my WACP with the 28-70 zoomed in for some shark photos but post-trip image review showed softness outside the center of the frame. I guess I need to stop down a bunch when using the WACP at 70mm even though its sharp on the edges wide open at 28mm. I dont remember the same issue with the 16-35 on an 8.5 dome but maybe I wasnt paying as much attention or was already stopped down a bunch even at the wide end. Anyone have any advice for how much to stop it down or how much of the 28-70 range is supposed to be usable on the WACP?

What are you shooting at f stop wise? and how does your copy of the lens perform at 70mm on the surface at those f stops? A not so good copy is always a possibility, assume this is the old Nikon 28-70 f3.5-4.5, maybe purchased second hand?

With my sample of the 28-70 I would avoid anything wider than F5.6 whem zoomed at 70mm. F5.6 is ok, F8 preferable. 

Have you thought about using the Sea and Sea 77mm Internal Correction Lens?  It works wonders for my 16-35.