Help choosing diopter correction

I am finding I do best if I use live view without my glasses. My vision is pretty sharp out to about 20 inches. The problem is, I can't get the viewfinder adjusted to work without my glasses. I have it adjusted full negative, which is close, but could use more.

The 1DS-mk3 spec is +1 to-3 dipoters. B&H lists a -3 diopter eyepiece. This ought to give me -2 to -6 which I suspect would cover my range of adjustment.

Does this cut light transmission? Anyone using one? How do you like it? Anything wrong with my understanding of how these work?