Just as a note: If you use these, don't be blinded by them. A lot of RPGers rely upon them, but forget to ask themself when they are useful and when they are misleading.
For instance, if your foe has DR that you can not pierce, you need to reduce your effective damage by the DR when making your calculations.
Another situation: If your foe is low on hit points, overkill damage is pointless, so power attacking, even though it will increase your damage, is a bad idea.
Another thing not often considered: If you expect to take AoOs, cleave off of high BAB attacks, etc ..., you need to factor those extra attacks into the equation when considering how much to power attack. If your attack routine is oing to be +20/+20/+20/+20/+20/+15/+15/+10, you end up with a far different optimal PA setting than if you have an attack routine of +20/+15/+10. In general, if you're going to be leveling foes or taking AoOs, it is often wise to bump up the PA a bit in a full attack routine.