I think it's fairly obvious that the final focal length is what matters. Which makes me think that I will need a different mask if I'm using a Powermate to reach 2000mm!

Fortunately for you, the 555mm vs. 444mm difference is well within the margin for error he posted. 3 x FL / (150-200) = 8-11 mm at 555mm, and 7-9mm at 444mm, so 8-9mm will suit either.

- Gus