It seems that a general consensus from feedback thus far is:
1) It is difficult to tell what the site does, and
2) Once that's figured out, few people stay because the site doesn't seem to solve a problem they actually have.
You mentioned iteration; What do you mean by it, specifically? Have you made substantive changes in functionality? Or have you simply added "most requested features"? The difference here is important, as it may be the difference between finding a problem that needs solving and adding resources to a solution searching for a problem.
My recommendation (for what that's worth) is to engage in a bit of customer development, the initial goal of which is to find who has the problem that you feel like you're solving. Hasenj noted that the calculations seem very simple (given, many contributors here are programmers and/or math nerds). That said, you do not know how many of your actual customers may be thinking the same thing.
However, if you can find (and by find, I mean specifically target, talk to and learn from) a group of people who consistently benefit from your service, you are in a much better position to move ahead. Effectively, this group will act as your cheering squad, providing feedback upon which you can grow and word-of-mouth upon which you can ultimately scale.
I consider this to be much more important than the design (although that is important as well). While better design may attract people to stay longer and/or use your service for the first time, it will not help with retention unless you're solving a problem that people legitimately have. Design doesn't provide an accurate indicator of scale-potential; Function does.
You may have known all of what I just told you. Hell, you may have already done it all. If, however, it was new to you, I would suggest checking out Steve Blank's book The Four Steps to the Epiphany, found here: http://www.amazon.com/Four-Steps-Epiphany-Steven-Blank/dp/09...
Best of luck!