You can set up percentage based bracket orders within your order Presets. In TWS, click on the Edit menu option followed by Global Configuration. On the left side, expand the Presets section by clicking on the plus sign next to the word presets. Next, click the product type you wish to set up Presets for (e.g. Stocks, Options, etc.). The Presets window will populate on the right side.
Click the "All possible field" button at the top of the window to expand all of the sections within Presets. Scroll down to the "Target Order (Profit-taker)" and "Attached Stop Order" sections. Here, to the right of Limit Price and Stop Price you will see two fields: one where you can enter in a number and the other a drop down menu. From the drop down menu, select "%" to set up a percentage bracket order. In the field provided, enter the percent offset that you want for your order. You may also adjust what figure you would like to use to calculate your offset from the first drop down menu to the right of Limit Price and Stop Price, respectively.
Once you have set up your bracket order Presets as you wish, click the Apply button followed by OK to close the window.