[Fondueset]

I'm between trux and cdavis on this one. Dry Statics are relatively safe and will help you be able to sense yourself and relax. However; NEVER expect your open water bottom times to be close to your dry static times. My best times are generally very easy dives, with plenty left over when I surface, and no attempt on my part to make them long dives. The fact that practice breathholds staying down in open water has helped extend times is not a surprise. Black-outs almost allways are. There are two major aspects to good bottom times - comfort/relaxation in the water and breath-hold ability. I really don't think you should work on both of them at the same time.
My times increased 40% when I focused on deep, mostionless, relaxation during surface intervals. At depths under 8m I sometimes dive with half a lung-full or less - to compensate for being under-weighted. With the above recipe my times are still better than they were before - and nearly as good as with lungs full.

Take it easy, have fun - don't have breath-holding contests underwater. My answer to your question is - practice holding on land if you want to. Work on deep relaxation and energy-efficient swimming (minimizing muscular effort) in the water. Place your limit - in the water - at when you start to feel uncomfortable - dont' push it. You should be able to glide gently to the surface with a smile on your face.