It is very simple to calculate the exposure time when using ND Filters when the light is constant. Long exposures captured during changing lighting conditions make it much more difficult to calculate proper exposure.
Let's say that you are preparing to shoot 10 mins after sunset. The lights is still good. Your exposure without filter is about 1 second and if you want to use 10 stops filter, you would need to take about a 15 minute exposure. But during those 15 minutes the light is dramatically changing and it is a lot darker by the end than at the beginning. That means that your calculated exposure will give an underexposed image. If at the start of shooting, the normal exposure is 1 second, by the end of shooting it will be, let's say 4 seconds. Further, the function of that change is not linear. So it is difficult to just average the exposure time between start and end of shooting and make calculations.
Are there any predefined formulas, based on personal experience, for calculating exposure time in those hours of the day, when light is rapidly changing?