When you mention withholding tax, are you referring to UK or US tax?
If you are referring to UK tax, then the share options you have from your US employer are probably part of an option scheme which is not an approved scheme as far as HMRC in the UK is concerned. You can read about the different types of share option scheme here, here and here.
As the scheme you are participating appears to be un unapproved one, when you exercise the option, you will pay income tax and national insurance contributions on the difference between the sum of what you paid for the option when it was granted, if anything, and the amount you paid for the shares on exercise and their open market value on the day of exercise. So, you will probably pay at least 40% income tax and 2% in NICs on the paper profit. The ETRADE calculator has assumed you will pay NIC at 12% but that is only paid on earnings up to £41,865. The excess is charged at 2%. As you earn around £100,000 per annum, you are already into the 2% NIC band.
If you exercise the option and hold on to the shares, then your cost for Capital Gains Tax purposes will be the sum of what you paid for the option when it was granted, if any, the amount you paid for the shares on exercise and the amount on which you paid income tax. Only any increase in value of the shares over and above that figure will be liable to CGT when you sell them.
If your income is above £100,000, you start to lose your personal allowance at the rate of £1 for every £2 of income above £100,000. The share option gain will, therefore, have an impact in that regard.
I hope this helps but let me know if you have any further questions.