You may download the feature limited trial, and evaluate the Screen Protractor for as long as you need to. By purchasing the full version you can unlock the software to measure the full 360 degrees, and use the advanced features.
You may also configure your own Custom Units using the 'Configure Units' menu option. Choosing this option will bring up a screen which allows you to choose a Custom Unit, rename and set the scale. You may also set the zero point, enabling easy relative angles to be calculated.
The Protractor has a fully 'skinnable' interface. This means that you may download new skins and change the user interface. From aesthetic to technical you should find onscreen solution that suits your needs.