I use (1 - STR)*FTR + STR.
It comes to the same result, but feels more logical to me. The first part calculates the effective Federal rate after subtracting the State Tax deduction, and then adds in the State Tax Rate. To me that reflects how you work out your taxes better than the formula in the article linked in the OP.
However, neither formula applies to everyone. As mentioned in other posts, it does not apply to people who do not itemize deductions. Among those who do itemize, it is still inaccurate for people who:
[ul]
[li]Have deductions reduced because of high income[/li][li]Are subject to AMT (state tax is not deductible for AMT)[/li][li]Would not exceed the standard deduction without taking into account state tax[/li][/ul]