I'm glad you asked this, I'm currently slightly worried about getting pregnant from precum only and while I realize that 27% is a pretty high number, I still feel better that there's a 77% chance that you won't. I would think that you'd probably have to time it just right on your ovulation cycle for it to be a big risk.

Still, a better form of protection is always preferred. Sometimes there's not time for that though, right? =P