Changing ISO mid-flight to get the right exposure?

[marceldacs]

I use ND filters and film on manual in d-log at 4k 30fps and 1/60 shutter speed. I usually try to keep my ISO at 100, but frequently have to switch to 200 during my flight. This just makes it annoying because then I have to remember which ISO the footage was shot in when I'm doing my noise reduction later in post. My main question is whether its ok to just leave the footage slightly under or overexposed, as long as my histogram shows that no data is lost at either the left or right peaks? Also, what is an acceptable range to keep my exposure value? It isn't always feasible to get it to exactly +0.0Ev, so what are the highest/lowest values before I should consider changing ISO or swapping the ND filter? Lastly, if I have a very bright sky but dark ground, is it better to get the exposure right for the ground at the expense of slightly overexposing the sky?