I've been meaning to weigh in here, but haven't had a chance until now. I purchased your previous copy of this lens last month and experienced similar issues on my k5 and k10d.
Based on my experience with the copy I have, I very much doubt this is due to technique, or if so only in part. I've taken this lens head to head with my 50-135 zoom to see if the macro can take its place (my original intention when buying the macro). I rarely have any problems with center-point focus on the zoom, so I suspected the lens from the start. This is the first lens I've had with an issue like this and it was very frustrating at first.
It's kind of hard to find information on this particular issue with this particular lens, but I did find a few things of interest. I read a suggestion that a macro lens such as this is optimized for the closer focus ranges and can therefore be more prone to being off for longer ranges. Seems pretty logical, I guess. Maybe it isn't recognized as much because it's not a lens that is used as often at the longer ranges?
I seem to have had success with a +3 micro adjustment after some pretty simple testing. I sat in a chair and chose a door knob about 9 feet away (just short of infinity focus) for my center-point focus subject, taking 3 shots: manual focus, auto-focus, and live view for each micro adjustment I tried. I don't suppose the manual focus is necessary here, but did it out of curiosity. Live view is going to be spot on, unaffected by the focusing issues, allowing you to directly compare each adjustment setting between the two (on the camera lcd then on monitor to be sure). My first guess was a +2 and +3 happened to get it right. After the adjustment I was getting pretty consistent results at all distances.
I may not have it exactly right yet, but will fiddle some more. Let me know if you have any questions and I'll pass along anything else I find.
Here's an example from a couple of days ago, post adjustment: