Duke M.
Veteran
A ND .3, .6, .9 and a 1.2 will do the trick, cause you can use the internal ND and tweak to the desired stop from there.
I was doing some testing to see how the internal ND's stacked with filters in front of the glass do. I'd always prefer to use a single ND than adding more. As for me, I'll be getting some higher density ND's.
How did your testing come out? I was figuring on Timur's idea of using the internal ND and putting on the external ND to tweak it. That was one of the reasons I chose Schneider filters. They are good enough you can zoom through them without defects showing up compared to cheaper filters that I tried. The cost is worth it.