Thank you!!!I need to organize the data and I'm feeling lazy right now but this was taken from a standard 75g tank... the readings for 9" offset 16"/18.5" depth (the other light is raised up on feet...) are partially blocked by some stem plants... oh well. Depends what you consider medium but maybe with lids and especially at the front/back it may be "low". The feet on the fluval give almost no height.
From what I am reading on your chart with all lights on you are getting a reading of 64 PAR at 19 inches with an offset of 9 inches. Most of my plants will do fine with low light but do better with medium. The only ones that really need the stronger light is my dwarf hair grass mats which unfortunately set in the front left of my tank. They would be at 21/22 inches offset between 3.5 and 9 inches from center. I am currently running four 30 watt t8s. A rough guess would put the PAR around 30 offset 9 inches at a height of 24 inches. If my math and curve are right. It would be on the cusp of what they like but I may be able to get by with 1. I was thinking about getting 2 and setting one towards the back of the tank and one towards the front but if I can get by with one I would rather invest the $160 for the second light on something else.
Bump: I have Italian Val, amazon sword, crypto Wendi red, bacopa, moneywort, wisteria which will all do fine with a lower light but the dwarf hairgrass is the one I am worried about. I don’t use co2 but I do use limited frets, root tabs and flourish as well as flourish excel for the carbon.