This is a preparatory necessity when beginning any painting, IMO. I like to find my lightest lights and darkest darks right at the get go. I've found a useful tool in my photo editor that makes this job very easy. BTW, I use Irfanview but I'm pretty sure every photo editor has a similar feature. I use 'decrease color' set on '2' which automatically greyscales my reference photo and reduces it to just two values. I've saved a few screen caps so you can see what I mean.