if your diy lights are powerful, you may need a little less light (more height could do it or slight dim -- height ismore wasteful but more photons probably hit the lower areas too... more list to absorption/missing plant also)
if your lights are good for 12/12 18-20" away, then you need 66% of that power over 18 hours.
if you know umol/s of your lights, it's easy to calculate DLI (or use a phone app and take several measurements across canopy.) umol/s divided by m^2 of coverage = X then reference a daily light integral table (google image) for 18 hours and the PPFD value you just calculated..
that's just total dli produced by light. The distance from canopy will usually be dictated by required coverage (geometry), but extra inches needed if too intense for 18 hours per day..
i noticed diy 240w lights and usually these lights don't cut as many corners and are fairly high-end. My 240w lights produce nearly 700umol/s of PAR. (700 / .84m^2 =833 ppfd at 18hours is 54 DLI ... i'd definitely give my plants light burn at 100% from my normal distance in bloom. Dim light slightly but also raise it 2-4 extra inches.
invese square law -- 1/d^2 * 100%
so if you go from 18" to 24" (1+ (24-18/24)=1.33) --Then, 1/1.33^2 *100% = 56% of previous intensity for entire area of coverage at new plane 6" further away than 18" original height -- this is more complicated by many single points of light adding together.. your central par values under light will not be 56% of what it was, but it will spread out to edges more. The total light /area will add up to 56% on that plane. the area enlarges in proportional manner.. again tent walls are being ignored here too. but, this is cause and effect of what is going on regardless of it getting messy. If you have your canopy coverage evened out, it should work effectively to reduce dli at canopy. it does not take much proportional distance change to make a big impact either.