I just finished two chairs that sound like your project. I cut the curve of
the crest rail from three inch thick stock and drilled out the holes with a
hand drill. But...I used a piece of plywood with holes drilled first to set up
the right spacing of the spindles for the final "look" that I wanted. Actually
I made several trial runs before I found the right spacing. Once the right
spacing is determined, it is easy to transfer the hole location to the actual
crest rail. One chair I made the holes were too close together so the final
"look" was like a barrel back chair. It the holes are farther apart, the
affect is that of a "fan back" chair. In my case the spindles start at the seat
back and run through the arm piece and end at the crest rail. I tried to take
my crest rail hole spacing from the spacing of the holes in the chair seat, but
that did not work well due to the curve of the overall chair back. Confused
yet? If this is not clear, let me know and I will try to give more detail.
If this does not work for you, you can always laminate a curved crest rail
using 1/8inch thick stock glued up on a curved form. I did this with my arm
piece and it worked fine. Most who look at it can not tell it is laminated.