Yes you can, and it will make the calculations of your returns more accurate.
The default setting for your prescribed investor tax rate is set at 17.5%.
When you're logged in:
click on your name (in the top right-hand corner) to access your profile page, and
in the third box you can update your tax rate by selecting from the drop-down menu.
This website is for general information purposes only. It is not an endorsement of the companies or asset classes presented. You should talk to a qualified financial advisor before making any investment decisions.