Lets break it down.
Turn the light ON when the value drops below 400, and OFF when it rises above 500. It’s a good idea to keep a buffer between ON and OFF so your lights don’t flicker. These numbers are just examples — you might prefer OFF at 450, 550, or even 600. It’s up to you. You can set this up with my 
Smart Light blueprint using Trigger - Ambient for this.
Once the light is ON, you also want to control its brightness. For that, set up Dynamic Lighting (option 1 or 2, depending on your needs). Note that Dynamic Lighting only works with entities in the light domain, not switches. The light must support brightness adjustment. If you can adjust it in the Home Assistant UI, then the automation will be able to adjust it too.
Blacky 