This is the recipe for DLI
Ppfd x (3600 x your light hours) / 1,000,000 = DLI
At 500 ppfd … 500x (your 18 hours x 3600 = 64,800) = 32,400,000 / 1,000,000 = a DLI of 32.4
You multiply 3600 because it is the amount of seconds in one hour… and divide by 1,000,000 because it is the number of micromoles per mole
I use 500 ppfd as an example because it takes 30 moles to grow quality buds and 500 is a good round number to shoot for as a min ppfd and it gets you above the 30 m min @ 32
I’m thinking that you might be getting an off ppfd because you have it set for 4000k the app I use has a natural daylight 6500k setting and I was told that at that setting the app was most accurate measuring led but I don’t know how close most accurate is 
I’ve messed with it a little I has another setting for red…blue…and white I get the same readings as the daylight setting 
…some day I’ll get off my wallet and buy a meter but I’m not convinced 500 bucks is worth knowing my ppfd.
Hopefully this helps you out… I finally got tired of stressing about the numbers and went back to growing
Edit = I just wanted to add do the math on the sun outside my app reads 2000 ppfd in my back yard where I have a happy little plant not burnt at all