If we take a ball-park 30 feet per hPa then from msl to FL70 = (7,000/30) = 233.3hPa.

Pressure at msl in ISA = 1013.25hPa so at FL70 it will be approximately 1013.25 - 233.3 = 779.95hPa; so look for an answer close to 780hPa.