We need to first establish how likely people in various IQ ranges are to be able to understand the concepts of LW: This is necessarily guestimate but it almost deserves its own debate. We can't just simply say minimum IQ is 120. I think its more accurate way would be to check out literature on what IQ ranges tell us about the likelihood academic success and being good at math (in more quantitative ways that just saying "It helps").

My suspicion is that for a lot of LessWrongers have weirder brains than raw IQ will capture. This is part of why we tend to be underachieving geniuses. I took WAIS III when I was 17 and got a 150 verbal IQ and a 120 performance IQ. That averages to a 135- but that number isn't actually a very helpful predictor of future performance because the two scores are so different. This kind of thing gets labeled a learning disability, they didn't even bother writing down the total on my test results. I suspect a lot of people here also have weird brains with strengths and weaknesses not accurately conveyed by raw IQ.

ETA: Which isn't to say I have a better way of estimating a potential user base.