When travelling light I use a Nikon D5100/10.5mm and R1 on a free-standing monopod. I shoot 6 round (0 degrees) plus zenith (either 90 degree on the head or hand held at roughly the NPP position depending on the light level) plus nadir. I use the latest PTGUI pro for stitching.

I'm getting inconsistent results with the zenith blending. I'm aware the NPP moves for the zenith shot and that viewpoint correction is the way to deal with the issue but how do you go about it when the zenith isn't flat (which I believe is a requirement for it to work) or when it's featureless (blue sky) and it's impossible to find matching detail?

This one has me foxed as sometimes it works and other times it doesn't. Is there something obvious I'm missing?