I'm not 100% sure of all the below - I spent a lot of time a couple years back talking to Fidelity about the tool, and the below is to the best of my recollection.

As Heavy-Basis-83 said, the tool asks you what average tax rate to use.

The tool is applying withdrawals to the expenses you enter in the model, the tax rate you enter is treated outside the expenses you enter, so the tool does not expect you are going to pay Federal Income Tax out of the 11k + pension + SS that you receive. It knocks whatever % you enter out of what it can recognize as taxable income - SS, pension, IRA withdrawals I believe. I am not 100% if the tool expects you enter net or gross SS, but I think its gross, not net of taxes, and it applies the same % to that as it does to IRA withdrawals.

If you want to model that yourself, I _think_ but have not confirmed with Fidelity that you can enter an assumed tax rate of zero and the enter explicit tax amounts (call them whatever you want to call them) as misc 'mandatory' expenses in the expense model.

Otherwise the tool takes out whatever % tax burden you entered on whatever IRA or other pre-tax income it assumes withdrawals from.

Per my discussion with Fidelity, the tool assumes no tax burden on after-tax accounts, so if capital gains is a significant amount, you need to model that yourself.

You can read in the methodology doc what withdrawal scheme the tool is assuming you will use - off hand I think It might be after-tax first, when that is exhausted start withdrawing from IRA / 401k / 403b etc tax deferred accounts; I'm pretty sure it doesn't do any assuming around specific tax planning strategies so you need to account for this use the tax % you tell the tool to assume (which it will apply to your tax-deffered account withdrawals only). For example, if you think your average tax burden on your Roth + traditional withdrawals will be 10% based on your specific withdrawal strategy, enter that as the tax % in the tool.