You can do what some of us do here to get close lumen numbers from flashlights.
Texas_Ace BLF Calibrated Lumen tube / Sphere No math skills needed - Several spheres still available Lumen Tube
WTS: Calibration lights for DIY integrating spheres / lumen tubes - 67 € - Calibrated lights
Adjust the diffuser’s to get the reading as close as posssible with both Calibrated lights.
This will measure OTF lumens.