I’m inexperienced; please help.

"You need 90% of the power (9/10 is inverse) for 20hours to hit the same DLI as stated above." -- guess i assumed you'd know to multiple 90% x 43% or 90% x 67% - to convert values for 38.9 DLI in a 4x4 or 5x5 area over 20 hours. So you can convert these estimates to whatever hours of operation you use.

This assumes the spec sheet is accurate, which may or may not be the case. If you do this, it should at least be clsoe and youu can fine-tune it from there. 18" is a good hanging distance for this light. You don't sacrifice overall intensity and fairly even from center to edges. If you go higher, you'll get a more even coverage but may need a few more watts. this would be slightly wasteful and may or may not justify the benefit it provides. Worth a little trial and error since you have alot of extra watts to spare with that light and i'm assuming a 4x4space. 2.9umol/J x 730 watts = 2117 umol/s at 100% power. for the most part the dimmer perecntage will be a proportional change to umol/s. 16 sq ft? This is ~1.5m^2 25 ssq ft? This is ~2.3m^2 35-40 DLI at diode is all you can give per day with ambient co2 in a typical household. This assumes reflective walls and a ~18" give or take hanging distance. Hanging distance is more about how even the light is from center to edges than anything else, but light spreads out according to inverse square law, so it can have an impact over short distances relative to total distance from canopy. The longer the distance from, the less a small change will impact intensity of light in any one location. Providing 35-40 dli from further away will give greater penetration, BUT will be more wasteful with electricity. There's a happy zone... 18" is my best guess for this light. So, we have the area in M^2. Now we find our target PPFD and then convert that to umol/s for your area. 18-hour operation for autoflower or vege phase of photoperiods: 600 ppfd provides 38.9 DLI, says any DLI reference table. 600 ppfd x 1.5 = 900 umol/s needed 600 x 2.3 = 1380 umol/s needed 12-hour operation need 900 PPFD: 900 x 1.5 = 1350 umol/s 900 x 2.3 = 2070 umol/s ------------- So, 18h and 4x4 needs 900umol/s. 900 / 2100 = about 43% 5x5? About 66% power needed, but you may want a higher hanging distance and that may require you to give 5-10% more power to hit max DLI. These are ballparks. You absolutely need to watch how the plant grows and adjust. It will tell you if it is getting too much light-- more so when it is adding growth nodes. The distance between growth nodes is the indicator for too much or too little light. Once in flower, you should have that figured out, but since that may not be the case this grow, just keep an eye out for light damage on leaves -- won't progress beyond top leaves, chlorosis and other damage is possible. It may takes several weeks to see damage if only giving slightly too much, so take notes of power and dates etc... you may need to look back 2 weeks to correctly assess any adjustment. This light is powerful enough to handly a 5x5 of photoperiods and a 12-hour operation. It's an absolute beast to use on autoflowers and an 18-hour operation. That's why it has to run so low. The hours of operationa are 1:1 inversely proportional to power needs. If you increase hours of operation by 150% (3/2), you need 67% (2/3) of the power. There's no need to give more than 18h of light, but if you wanted to run at 20hours, that's 20/18 or 10/9 longer. You need 90% of the power (9/10 is inverse) for 20hours to hit the same DLI as stated above. Same with giving less. With this light you could run fewer hours. This would be beneficial if you need a little extra heat or to keep a larger bulk of operational hours during cheaper times of electricity. DLI is what dictates yield. Giving the same DLI over 12-16-18-hours etc will all have equivalent results. You have everything to extrapolate an answer for any situation from the above. As i said these are balparks and will need small adjustments from there based on what the plant dictates. I tend to err on high side rather than low side for lights. Can always dial back or add an inch of hanging distance, but you can't manifest more light from nothing.