safety stop timer

Got your computer, the DG02, and the safety stop countdown function doesn't work really well for me. It starts counting down the three minutes at 20 feet - I may still be diving/swimming around then and want a proper safety stop at 15 feet for 3 min because that's what I told people we will do. By then I'm in the middle of the countdown and won't have any time displayed that would allow me to time my three minutes. How do I avoid having to time the SS on my other watch? Hard to watch both wrists simultaneously.