Believe a standard atmosphere table.
The actual ratio of altitude change to pressure change is closer to 27 feet per millibar.
Using this, you get a pressure altitude of 27 * 21 feet, or 567 feet.
Compare this with the pressure at 570 feet in a standard atmosphere table: 992.6 mbar.
http://www.digitaldutch.com/atmoscalc/table.htm
It is amazing how poor the study material for would-be pilots often is.
Cheers,
/Fred