In addition to the information here I found this http://www.wekeepyoucycling.com/en/p...ring-tool.aspx

This bicycle shop claims that after you measure your sitzbones they will help you chose a saddle.

I can't help you with the proper placement issue as I've not been able to get my saddle back on correctly after I've taken it off. Therefore, it's a job that I delegate to my bike fitter.