Every rec computer that I have paid attention to (which isn't a huge amount) the SS counter will start at 20' if turned on.
As a DM, I choose 15' because if I choose 20, I'd have divers at 25 and 15', if I chose 10', half the people would be at the surface....... It's like splitting two evils, 15' seems right in the middle and a good choice to keep holiday vacation divers incheck - no offense meant. I don't know if it happens, but I don't want someones computer being mad about not completing a SS, I want everyone that has a timer running to reach complete before we surface - that means you have to stay roughly between 20' and the surface to keep it running - I usually gotta pee so I'd rather not yack around......
From a tec point of view, I set my last stop at 20', complete half of it at 20 and move up to 15 to finish it, I never go to 10'. Setting it at 20 and bumping to 15 verses setting at 10 and never getting there theoretically maybe gets you out quicker than your initial TTS, not later - just one of those stupid detail things. I don't do crazy dives, I can watch my surface GF and it seems comfortable to me......