Issues with scale

I am doing the galvo lens correction procedure on my 300mm lens, and I’m having issues with scale. If I set the parameters to 100% for scale, and draw a 300mm box, the projection is much larger than that. Now, I know this is because I set the scale to 100%, but my question is do I need to scale my projection down to 300mm, or adjust my scale %?

Which one?

:smile_cat: