Assuming I'm reading this chart correctly, and the asterisks mark where the 1A classification split between 11-man and six-man, the six-man cutoff was 95 from 1986-1994. Then from 1994-2004 I'm not entirely sure what the number was. There are two lines with asterisks on those years, but assuming the lowest was the six-man cutoff, that means for those 10 years it went down to anywhere between 79 and 96, depending on the meaning behind the asterisks on those years. From 2004 to 2014 it was 99.9. Since 2014 it's been 104.9.
It did go up drastically from the 1974 to 1976 alignment but, unless I'm missing something, which is entirely possible, it looks like the cutoff for six-man has only risen by 10 in the last almost 40 years.