FX Draw has included compass-like tools for many years, but they have always been a bit difficult to use. The latest version of FX Draw (V7.003.7) now includes greatly improved versions of the compass tools.
When you choose either the compass circle or compass arc tool, the top toolbar now changes to show you the current setting for the compass radius.
The compass radius is currently set to 20mm and any compass circle or compass arc drawn at the moment will be drawn with exactly that radius.
You can explicitly change the radius to any measurement you want, but sometimes you will want to set the radius to something defined in the diagram. For example, I might wish to set a radius to be the length of AB.
Rather than breaking out Pythagoras, I can measure my radius on the diagram. Push the compass button on the top toolbar.
You can now draw a standard circle with the centre at A and moving out to B. Once the circle is drawn, the measured radius is automatically entered into the Compass Radius box, ready to be used on subsequent circles.