I'd expect a lot of people to leave the question blank if there are other IQ questions. I'd also expect a lot of people to work backwards from scores, more or less formally -- I don't think I'd expect many people to actually do the math or track down a normal distribution calculator, but thinking like "well, I got 99th percentile on that standardized test in high school, so I'll say 100" would probably be common.

To be fair, that's probably more accurate than what you'd get by counting up the number of people you know who're smarter than you.