DAD 360nm how to use it?

Good, so then everything in the document applies to your DAD.

And here's the user manual for your detector:

https://www.agilent.com/cs/library/usermanuals/Public/G1315-90015_DAD-MWD-CD_USR_EN.pdf

Also in order to use 400nm ref wavelength I had to turn on the visible lamp.

Also in order to use 400nm ref wavelength I had to turn on the visible lamp.

I don't see why the tungsten lamp would be required at 400nm. You might have set a "Vis lamp required" somewhere in your CDS or one of your settings points to a higher WL.

Finding a suitable reference wavelength and reference bandwidth should be part of your method development. Please go over section "Sample and Reference Wavelength and Bandwidth" in the manual and see the pages for reference selection in the DAD document, I shared.