The flaw with your calculation (although not your logic) is that the rate of pressure change with altitude is not uniform; it is around 27hPa at sea level for a few thousand feet and then the rate increases with altitude. 300 hPa and 30,000ft is part of the ISA standard, along with standard pressure at other altitudes (which I'm afraid I don't have to hand). Any pressures within these standards have to be extrapolated.
Cheers
Whirls