Okay, I'm an idiot. Just realized I was reading my flow meter wrong while doing my daily volume/leak check. There are two measuring units on it (LPM/SCFH). I've been reading the SCFH units, mistaking them for LPM.
So when I did my first few practice runs, I claimed it was at 20 LPM, but it was actually 10 LPM.
When I did my last tests, I claimed it was at 30 LPM, but it was actually 15 LPM.
At 15 LPM, I didn't suffocate/hyperventilate/panic and felt the tingles and lightheadedness, so PPeH had it correct about the flow rate IMO.
Still gonna follow the recommended 25 LPM with the 3M hood for my final run. My backup cancelled recently and I doubt I would be able to get an EEBD hood prior to the end of the month so I'll just have to sit tight with the 3M hood.