Bottle and bucket is the simplest method. And a accurate method IMO. most everybody uses this method to meassure production so it probably best way when comparing outputs.
http://www.dwyer-inst.com/PDF_files/RM_iom.pdf
These work good and not very expensive. (You must correct the # for the gas you’re measuring.)
For HHO, multiply by 1.53 for this type of meter.
Correct for temp also if you really want the true reading (This applies for the bottle method also)
Air at 2 LPM at 120 degree = 1.92 at 85 degrees
Air at 2 LPM at 150 degree = 1.86 at 85 degrees
I don’t know if temp correction is much different for HHO
If some knows this correction please let us know
Correct for pressure if their is any.
Then if you can figure out a way to factor out steam you'll be spot on