From a source such as HN on something targeted like this I'd expect it to be much lower (<50%) the rate observed may be due to slow load times or landing page design or some other factor. There definitely seems to be some disconnect between expectations and what was found on the other side of the click.

Possibly someone that did a comparable launch on HN could give exact figures if they're willing to part with them then you could compare notes to see what works and what does not.

These can be tricky things to dig up, sometimes eye tracking is a solution to figure out where the disconnect is. There was a HN start-up working in this field (GazeHawk) but they've been acquired by facebook.