Used to blow the old mechanical vitalagraph of the scale capacity wise but could never get better than 82% in the 1st second, 90% pass?
Got 130% on lung capacity,of average for hight/weight? on the new electronic vitalagraph. Had already passed my part 3 when the vitalagraph was introduced so all I had to do was sign the medical disclaimer
I'm not a doc, nor an expert on these things, but I'm sure that one breath out will measure the volume of your lungs and then the vitalagraph measures how quick you can empty them. Try practising blowing down a kitchen roll tube.
I have heard of this before, a lad who was very fit, and blessed with larger than usual lungs, and was shitcanned purely because he couldn't empty them quickly enough. You appear to have the answer to this problem now with the gizmo you've got hold of, so good luck!