Roland
The convention as I believe is that your Subs and Darks will always be the same length exposure, ISO and temperature as you know. Like H, I try and take some Darks at the beginning, then the middle and at the end to get an even spread of temperature.
I am not into the physics of how it is done, just know it works OK for me
, I have always been lead to believe that the ISO of Flats is of no consequence so long as they are the same size as the rest of all the frames. I suppose when you think about it the fast shutter speed of a Flat may well negate the use of a Dark Flat....