In doing recent research, I have discovered that the Formatt Hitech brand is no longer available in the 40.5 mm size. You have these alternatives:
When I initially looked into C23 filters, Gobe had only one choice in the 40.5mm size at that time... the ND 0.9. Now they have two different sets... one set with 3 filters of ND 0.9, 1.8 and 3.0 (3, 6 or 10 f-stops). The second set now available has 5 filters of ND 0.3, 0.6, 0.9, 1.8 and 3.0 (1, 2, 3, 6 or 10 f-stops)... barely more usable.
The other brand option is Heliopan... they have individual filter choices in 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.0 and 3.0 (1, 2, 3, 4, 5, 6, 6.6 and 10 f-stops).
When I initially looked into C23 filters, Gobe had only one choice in the 40.5mm size at that time... the ND 0.9. Now they have two different sets... one set with 3 filters of ND 0.9, 1.8 and 3.0 (3, 6 or 10 f-stops). The second set now available has 5 filters of ND 0.3, 0.6, 0.9, 1.8 and 3.0 (1, 2, 3, 6 or 10 f-stops)... barely more usable.
The other brand option is Heliopan... they have individual filter choices in 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.0 and 3.0 (1, 2, 3, 4, 5, 6, 6.6 and 10 f-stops).