If you're slipping forward, maybe you should leave the saddle forward? Seems to me that moving it back will just make you more forward on the saddle.

You also might want to try angling the nose up a touch. I actually prefer the saddle to be at what looks like a painful, nut-crushing angle; for whatever reason that's comfortable and stops me from putting too much pressure on my hands to keep from slipping forward. I am not, however, a bike fitter.

KJ