My folding to the river bet percentage is 50%. This seems to be in line with most other good players. Many times, I don't have a choice. I'm either forced or voluntarily all-in, or it is checked down. I certainly can't figure to fold to a check (although I have folded a few hopeless hands rather than show them.) I have also called a few hands where I was pretty sure I was beat but the bet was so small that I had great odds. Most of the time, I am quite surprised when I'm beat at showdown.
I have meticulously studied every showdown and, where I have made a few mistakes, most decisions I made I couldn't fault and I'm likely to repeat.