There was a study of this, showing that the the highest game shape increase occurs at ~60, though 50-70 is certainly about enough to keep GS from decreasing wit hsufficient probability. 48-90 seems a bit wide.
Someone more meticulous than me might have a link to a neat little graph of how GS ups were distributed.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."