The comments about backfocus are right on several points. There are several roadblocks to being able to get good results with the adjustments.
First, tackle the issue with AF. Then you can handle other steps that may be necessary. I've been through these steps several times with my two K10D bodies.
You'll need a controlled environment with which to test AF. You'll also need your fastest auto focus lens you can get -- the 50mm f1.4 works very well for these tests. Then get a surface that has a texture that allows you to understand when you can try to focus on something and compare results. These are what the focus charts are useful to do. You can also use a ruler set at an angle. Also, you'll need to know the color temperature of the light you are using and make sure that you repeat this setting. It does make a difference if you are under incandescent light vs sunlight.
Next, you'll need to access the debug menu for your K10D. There are several documented ways to get to this feature. If you have the original firmware (1.0) then there are basic key presses to turn it on. There are ways of putting a file on an SD card and thus by editing the text in this file you can toggle it on and off. I personally use a piece of windows software that allows me to turn on and off the debug setting when I connect via USB. The software that I use has instructions available here. If you want to know about the file on an SD card method, visit the Pentax Hack website. For what it's worth, you do not have to update your firmware to the latest version before doing this. In fact, if you have any SDM lenses in your collection, you'll want to not update past 1.30 so that you can flash back to a pre-SDM firmware to allow you to use these lenses with in-body AF motors (... this is another can of worms).
Once you've enabled the debug menu, start taking pictures of your focus chart. You'll use the debug settings to move the focus adjustment for the software. For me, I've found that under daylight (color temp > 4500 K) I use a setting of +140. For incandescent lighting (anything less than 3300K) I use a setting of +90. As I tend to shoot in different environments, I leave the debug menu on all the time, even though it will use the battery faster (Debug always shows on the LCD and thus it never goes blank).
Note that to really see if the AF adjustment is moving, you have to view the results on a full size screen. The camera LCD really doesn't have a good enough view to tell, especially when you're down to the last little bits.
As has been noted, these adjustments are for the body and will change the AF behavior for all lenses. If you have lenses that require a different setting, then you may have to remember this and change as necessary. I have been lucky in that all my lenses are able to use the same settings.
Once you've been able to get your AF working well, check under different lighting as noted above and learn what you'll want to set for when under that lighting. Note that if you use the flash-assist light from either the popup flash or a hotshoe flash, those will count as "daylight" temperatures. Otherwise, if you have dim environments that you'll be using flash, most likely you'll need the incandescent setting.
The reason why you'll need to have the different setting for the two types of lighting is that the AF software in most DSLR cameras is sensitive to light wavelengths in different ways. The good thing is that if you test properly, you'll know what to change to accommodate the light rather than be bound to just one setting.
Lastly, once you have your AF working well, take the time to learn what a scene looks like in the viewfinder once the camera thinks it's focused. If you see a scene that definitely shows your subject out of focus, then you may have to re-shim your focus screen. These shims are metal spacers that you can buy from Pentax that are in different thicknesses. By changing them, you move the focus screen slightly and thus what you see with your eyes you can then trust is actually in focus.
One final bit of information about the focus screen. The stock screen for these cameras (and many other DSLR cameras) show a DOF that is much bigger than what is really available. This is especially true when using lenses faster than f2.8. That can be the source of a lot of frustration when you believe your eyes and the camera shows otherwise. The solution for this is to get a different focus screen that is able to handle these lenses. These are available from many different suppliers. The main vendor for the high-quality ones is Katz Eye Optics. They can be a little expensive, but they make the difference!
Hope that this helps you in your efforts and congrats on owning a great camera! I have been shooting with mine since 2007 and still find new things to do with it.