DSLR as a light meter?

My D3400 obviously has a light meter built in. I have a need to verify that a white surface, about a foot square, is illuminated to a minimum of 1,000 lux. I have the two kit lenses which come with the D3400. Does anyone know of a formula to back calculate the lux level based on the shutter speed which the camera computes based on a given aperture or some similar method?

TIA,

Ken

I have an app on my phone that displays lux values. It is called Light Meter - Free, the company is WBPhoto in the Google Play store or whatever it's called for Android. It is free with a paid version option. A lot of not so useful to me at my level of photography stuff in it. Lol. There is pretty useful stuff in it as well. I hope it helps! Oh, my phone is a Samsung Galaxy S 9+ if it makes any difference.

Rob

Thank you FredKingston. That was exactly what I needed!

Thanks BeegRhob. I had received that recommendation from the vendor of the product I am working on. However, even though I have worked with computers since 1971 and have been active on the Internet since 1994 and have a veritable data center in my home office... when I leave the house I leave that behind. I do not feel the need to carry a computer with me nor have Internet access every minute of the day. I do not have a Chinese Communist Party tracking device (smart phone and don't want one.

If I NEED to make a phone call I have a flip phone - which is generally powered down. If I want to take a picture I have my D3400 and an FG film camera. Call me old school )

Ken

Do the math The video is helpful but his math as shown on screen is incorrect. Assuming the constants to be correct it is necessary to DIVIDE by the shutter speed to convert to Candels or Lux, not multiply. If you think about it the brighter the subject the faster the shutter speed would be. Other than that it gives me the number which I was looking for.

Ken