I am pleased that you have tried GrblGru.
But I’m not sure if I’ve understood your question correctly.
Actually, you only need to drop the DXF file onto the graphics screen using drag & drop, for example. Then the max. diameter of the workpiece plus a little safety should automatically be displayed at the bottom left in the “Start diameter” input.
By clicking on the little up/down arrows of the input, the value changes and the green tool path is updated with a slight delay.
If this does not work, please save the project file and make it available to me here.
Then I can better understand the problem.
Please also download the current version V1.1 from my website https://www.grblgru.com/