Tappers Dad
He be at FL230 which depending on atmospheric pressure could be (usually) 21500-24500' amsl which equates to 18200-21200' agl at KDH
Low atmospheric pressure means the aircraft would be flying lower above the ground, higher pressure vice versa.
For every 1mb difference in the pressure, the aircraft would move up or down by 30'
At that level, it would be flying on Standard Atmospheric Pressure of 1013.2mb (or 29.92 inches of Mercury - this is a more accurate measure where 0.01 inches change results in 10' up/down)