I recently bought some of this film and I am looking forward to making some filters with it. I have an 8" Dob that I want to make a filter for, but in doing my research I'm finding mixed advice about whether I should go full-aperture or not. I am currently leaning towards making a 4" off-axis filter out of foam board. It seems that, with something as bright as the sun, that should be plenty, but I guess I have to consider that relatively little light will make it through the film. The film I bought has 0.00001 transmission. This is not listed as a percentage, so I assume it is out of 1.0. Doing some rough calculation (which is probably incorrect), it should reduce the apparent magnitude of the sun by 13. If the sun is -27, then even with the solar film, it should appear to be -14 in brightness. Therefore, 4" of aperture should be plenty. This is why I'm leaning towards stopping my aperture down. Maybe even 1-2" would be enough?
Btw, the way I arrived at 13 was log(0.0001) / log(2). There might be a flaw in my logic here.