I'm not sure exactly what that statistic means. If my interpretation is off, perhaps you could clarify...
Mensa requires a Stanford-Binet score of at least 132 for admittance. Seventy percent of 132 is 92.4 - below average. So, I think what you are saying is that a Mensa-qualifier inherited a below average IQ from his parents. The rest was environmental.
Do I have that right?