DLI

Don't overcomplicate things as a beginner. If you don't run a CO2 System with a 1000Watt Lamp to get out every drop it doesn't matter. Keep a good distance like 25-40cm from the top bud to the light and you should be good. The Manufacturer also has (normally) some advice written down in the Manual Book of your Lamp if not look on their internet site/contact them or something like that. But as long as your plants look healthy and happy - you're doing fine even without DLI and VPD and and and. Hope this helps.

Keep it simple.........don't worry about DLI, VPD etc......it will not make any difference for the average home grower using average equipment. 18/6 then 12/12 is just fine......you will get better and more positive result and or improvements by concentrating on getting the feeding, watering and pH optimal.