I shot a roll of Aerochrome 400 today. Most photos were taken with a heliopan yellow 12 and rated at 400 with the filter attached. I metered at 400 without adding any compensation for the filter using the tiny light meter app on my phone. I've found this app to work well with other slide film. I also took a few shots with a red 25 filter which I've not used before. My yellow filter has a filter factor of 3=1.5 stops of exposure. The red filter has a factor of 8=3 stops. My understanding is that you add stops and multiply factors. Given that these filters differ by 1.5 stops and the film is rated at 400 with a yellow filter was I right to just increase my exposure by 1.5 stops? i.e. If my meter read 1/250 @f16 with a yellow filter, was I right to shoot at 1/250 @ f9.5 with a red filter.