Admittedly it's not calculated so it may be a stretch, it's based on the assumption that the vast majority of spam out there just looks for forms to submit without smarts (which is also why honeypots can be pretty effective, especially if you have a small website that nobody will take the effort to work around it.)

I've seen people report that they have reduced spam to near nothing already with just a honeypot, but of course I can't verify those claims.