For flats, I made a light box and as a light source led strips bought for a few Euro. Flats made with twilight or daylight as source where never good enough. There was always a slope. With no filter exposure time used for flats is maybe 40 milliseconds to exposure to 50% (=32000). With a H-alpha filter in place it requires an flat exposure time of about 1 second.
Since flats exposure is very short any light leaks from my manual filter wheel will have likely no influence. So in principle I'm only forced to take dark's under dark environment conditions. I also have put a towel around the manual filter wheel to prevent light leaks with some success but it is easier to wait till it is dark outside. If you have no open manual filter wheel, I assume you can take dark's in daylight conditions.
P.s. you could check your stacking software by stacking some flats as an image. If the resulting image is a single exposure value, the stacking math is working.
My simple light box design:
http://www.hnsky.org/light_box.htm
The second diffuser has been replaced by a Pearlescent Vellum paper sheet.
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