You could simply hire better employees rather than disrespecting your candidates. I wouldn't bother pretending that it has anything to do with "risk management". At best, you sound extremely cheap. At worst, it makes you sound like you repeatedly make poor hiring decisions and scramble to adjust for this.
Right, but as I said, after the three months, we settle the difference - and it's not 80% either, actually, more like 7%, but hey ho. We've never had anyone object, and it's pretty common practice.