What is effective tax rate that should be taken for firm valuation in US ?

Answer 1

Assuming it's at least a medium sized corporation, you can assume roughly 40% for the effective rate of federal and state income tax. This reflects the fact that the state income tax is deductible for federal purposes. However, state income tax rates vary widely, so you should consider the state where the investment will be located if you want greater precision.

Answer 2

The effective rate will vary with each situation. In general terms, effective rate is total tax divided by total income before any tax adjustments.