For anyone confused I believe you set cnc_scaling=3.93701 for a 100mm square steel sheet. I believe cnc_scaling is in inches. Correct me if wrong!
Ah, yes, exported G-code defaults to inches. You can change the "G20" in exported G-code to "G21" and then coordinates will be treated as metric. So if you want to make a 100mm hologram just set cnc_scaling to 100 and edit exported G-code to have G21 instead of G20 and it should work. The recommended workflow is to export your hologram optics as an SVG and import that into a CAM program that can generate G-code from the vector paths. That will give you more control over generated G-code than Holocraft does because the G-code export functionality is extremely minimal. Cheers! :]