One theory I've come up with is that the true value of the term 'beyond a reasonable doubt' is less in the specific percentage value, and more in that it makes for a significant difference in the evidence required to convict someone of a civil tort (in which they are merely required to compensate the harmed party) and the evidence required to predict that someone is likely to commit further criminal actions in the future (and thus it would be reasonable to take additional measures, beyond simple harm-compensation, to deal with the expected future threat); and that the 'reasonable doubt' standard is simply what happens to result in the right rate of convictions. (I wrote this idea up in more detail at http://www.ncc-1776.org/tle2011/tle639-20111002-05.html .)

The numbers you assign to "preponderance of the evidence" and "clear and convincing evidence" also seem badly skewed, though less so.

'Preponderance of the evidence' simply means more likely than not - 50%+1. (As I see TimS posted.) 'Clear and convincing evidence' is a level between 'preponderance' and 'beyond reasonable doubt', so without having found any particular surveys or statistics, I put it midway between the two.