I may as well post this here. I made a little video for someone else to show how I do my rough parabolizing to get the mirror to the stage of fine figuring. I find it works so well and produces a nice smooth zone free result. The lap is 8" with the edges pointed to help with blending.
This is my current 20" f3.8 project.
Here's a current Ronchi. Still a long way to go.
And here's a video of the way I work in W's that increase in size to smoothly dig the mirror down.
https://youtu.be/8SamXXGYKps