Since HM100 has only 1 setting for ND filter, I was wondering what additional ND filter should I get? 0.3 vs 0.6 vs 0.9. Sunny day in the mountains, some on snow. Any thoughts?

I would go with a N.6, which would give you an addition 2 stops. However, you should consider a polarizing filter instead which will also bring you down 2+ stops and give you the added benefits of polarization. (Deep blue skies and reflection control.)

The ND filter built into the HM100 is "1/10", which would be equivalent to about 3 and a half stops. As you may have seen in my field test in the desert I had to dial the iris down to F/5.6~F/8 to get a decent exposure in that bright sun. I would have preferred an extra two stops of room to get me in the F/2.8~F/4 range.

Thanks Tim!