Great question. I wish I knew! I know that when I look at my personal age group splits I always look a lot faster compared to my peers in open water. I think, as others have said, there are too many variables for a cut-and-dry formula. I would imagine that your comfort level in the environment and open water skills (or lack thereof) would make the biggest difference. I know some incredible pool swimmers who have been swimming most of their life - these men and women are like fish - and they can really kill a pool swim. Some are also great in OW, but they've built up OW experience. I personally love OW, not because I'm a super swimmer (I'm just decent I suppose), but because I feel like by learning to sight, to draft effectively, and trying to stay in the best line possible I can gain an advantage, or at least hang, with those who may be physically superior in the swim.