Get a cardboard box. Cut off the four flaps. Stack them. Sit on them with your knees slightly higher than you hip joints. Stay there for about 20 minutes (watch the news, although I doubt anyone can actually get twenty minutes of news).
When you get up, you'll have a perfect indentation of your ischial tuberosities, the sitbones. Measure center-to-center. If you can more or less place that dimension in a centered spot on the saddle, go for that saddle. If your sitbones get within, say, 3/4" of the edge radius, then it's a crap-shoot as to whether you will get a comfortable form out of it. My 17's are very much at the edge of that criteria as my ischials are a bit on the wide side. For me, a 67 is best for long rides.
You don't want to be sitting on the steel horse-shoe frame that supports the rear of the saddle. Ya just can't form that...