I'm using a cheap 10$ e3ay lux-meter, buddy, which shows quite accurate numbers compared to my calculations.
Twice as much as with smartphone apps, btw! I found an app that had a calibration function, but even with calibration to the e3ay lux-meter numbers, there are no accurate readings. My front sensor(I have a oneplus3) seems to have a limit, because it shows max. 35999lx, no matter how close I get to the light. I think the 10 bucks was well invested!
The sensor is mounted on a rod and the display unit is detachable with Velcro attached so that I can easily get into all corners. At 24" and 20w/sft, your numbers should be twice as high, estimated from my readings at least 30000lx!
You could try these tool to calculate PAR numbers at different heights from lumen and spectrum.
http://dev.edman007.com/~edman007/pub/par-dli-cal.html
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