When taking landscape shots, I really struggle to correctly determine what density of ND grad to choose. It normally takes a bit of trial and error rather than any scientific judgment.
I've tried to calculate the difference in light by using my camera's spot meter. I must admit, my mental arithmetic isn't that great and I find it hard to work out how many stops are between them.
When measuring from the ground to the sky, is it important to make sure that both measurements are taken from a midtone? What if there are no midtones in the sky?
Any other advice gratefully received!