How accurate is a Microlife peak flow meter in measuring FEV1?

Has anyone actually used a microlife peak flow meter AND a real medical spirometer to measure FEV1? I ask because I've found that by inhaling as deeply as possible before exhaling, I increase my FEV1 value by as much as 15%. And by taking measurements while sitting rather than standing the FEV1 decreases by as much as 20%.
What I'm curious about is whether the FEV1 on the microlife accurately represents the measurements found on a real spirometer.