Below is incredibly rare data of the total number of people in 1984 who scored high on the combined SAT.
We see that of the 3,521,000 Americans born in 1967, roughly 964,739 would grow up to take the SAT at age 17 in 1984. And of those who did, only 20,443 scored 1330+. If one assumes as the great Ron Hoeflin does, that virtually all the top SAT talent took the SAT in 1984 (and whatever shortfall was madeup for by foreign students), then those 20,443 were not just the best of the 964,739 who actually took the SAT, but the best of all 3,521,000 Americans their age. This equates to the one in 172 level or IQ 138+ (U.S. norms).
Meanwhile, only five of the 3,521,000 U.S. babies born in 1967 would grow up to score 1590+ on the SAT, so 1590+ is one in 704,200 level, or IQ 170+.
Meanwhile a national norm study found that if all Americans 17-year-olds took the SAT in the mid 1980s, not just the college bound elite, the average score would have been 787, so 787 implies an IQ of 100.
Armed with these three data points:
1590+ = IQ 170+
1330 = IQ 138
787 = IQ 100
Sadly, because the line is not linear, but rather positively accelerated (because of ceiling bumping on the SAT) no simple equation could be created, so I made a polynominal equating 1984 combined SAT to IQ (not sure how accurate this is since I only used 3 data points):
IQ = 114.13423524934914 – 0.06999703795283904(SAT) + 0.00006612121953074045(SAT)2