Your filter is an opaque IR filter, it has a filter factor of 16 and using it with SFX200 means you'd need to increase exposure by about 4 stops giving you an effective ISO rating of ISO12. Your best bet might be to check the composition, exposure and focus without the filter in place then open up/decrease shutter speed by 4 times when you add the filter. At such slow shutter speeds (generally between 1/2 and 1/15sec at f11 in normal lighting conditions) you'd probably be using a tripod anyway. f11 should give you enough leeway for any small focussing errors in most situations. SFX does have a fair amount of lattitude but it's wise to bracket one stop either way. I use it with an Ilford SFX filter which has the same filter factor as yours and I've found the best results tend to come from 4 to 5 stops over the meter's recommended exposure without the filter in place. It's a lot grainier than you'd expect from an ISO200 film but this just adds to its charm, it's a wonderful film and looks good souped in DDX. A lot of additional info can be found here. (pdf link).