Yes you have right – this is one of the reasons that I have moved to UI Theme roller and changed tottaly the concept of the grid in 3.5. In 3.5 you will not have such problems – moreover if you set 900px width it is 900px regradeless of the column width.
3.5 Alpha 3 is available now and seems very solid, so far as I have tested – I'm using a fair amount of the 3.5 feature/functionality and am happy that I made the decision in lieu of 3.4.
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