When you start receiving pension distributions your monthly benefit will have taxable and non-taxable portion. The main nontaxable portion is all of your own contributions that have not been deducted from taxable income in the U.S. In addition, employer contributions that have been reported on your U.S. tax return will be a part of nontaxable part. All this taken together will constitute the "cost" in pension.
The taxable portion will be calculated using the IRS calculator that factors in your cost in pension and your age at the time when you receive the benefits. This is not a simple calculation but after it's done once it will be easy to continue in the following years.
Tax treaties with some countries have a far simpler way of calculating the taxable portion: i.e. 25% of UK is non-taxable in the US.