Re: ASICMINER: Entering the Future of ASIC Mining by Inventing It
Best way to get an unbiased slightly under-estimate timing is U: after at least a few hours (a day or two is best) - since U: is what you get paid for 
Actually WU is what you get paid for. U is denominated in whatever pool difficulty you have while WU is denominated in difficulty 1.
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U: is exactly what you get paid for - accepted shares.
I am mining at 1diff so in this case it is even the exact share count when multiplied by the elapsed time and divided by 60.
If you are not mining at 1diff then yes you don't know how much you will be paid, but it still does tell you how many payable shares you have submitted.
The API "Difficulty Accepted" will tell you exactly how many 1diff shares you have submitted.