Focus within magnification

I bought a Pentax K5 with a Sigma DC 18-200, 3.5 - 6.3 lens in August 2011 and just used it to record travel etc and the photos looked fine. However, I was not a serious photographer and I did not enlarge photos much to check on focus. Occasionally I noticed that much enlarged photos were not focused but just reflected that any photo "over enlarged" would not remain in focus. I'm not sure now whether that is true after having submitted older and recent photos to a stock agency and to my consternation had them report that none of them were in focus.
Since then I have tried to take photos on AFS and MF and find that they are indeed blurred when magnified in Lightroom to 1:1 and above.
My questions are: to what degree of magnification should a well focused shot with my camera and lens still be in focus? I would like to think that it would be to Lightroom's 11:1 magnification and wonder whether its the camera or the lens or the combination of both at fault?

First thing to do if you haven't done so already is to calibrate the AF.

You be able to see from the photos whether they are front focussing or back focussing.

Take a bunch of pics at widest aperture in daylight and adjust the AD calibration ( custom menu #26 ) between +/-10 until it focusses correctly.

You're not going to see if anything is in focus or not in lightroom at 11:1 zoom as it's just blocky pixels ( each sensor pixel is 11pixels square on your monitor )

Smeggypants - seems so simple and don't know how I didn't discover this - my setting was on OFF! Allow me to say it - Idiot!
I have read up in the manual and will test at the first opportunity, but just one more thing please - will this also automatically correct when using the MF setting?

Thanks for explaining about Lightroom at 11:1 magnification, but then what is the largest magnification that one should use please?

If you mean 'will it affect the way the focus indicator behaves in manual focus' then the answer is 'yes', any adjustments will impact on this so your manual focus may become more accurate if you rely on the focus indicator rather than looking at the focussing screen in the viewfinder.

Regards
David

David,
Thank you for that information. Must say that I prefer to use AFS to MF, but Im glad of any measure that assists.
I'm currently identifying what calibration appears to be best (its not easy!) and I'm at the + end of the scale at about 9 or 10. My eyes are good as I don't wear glasses but even so I think I need a better pair! No doubt the resulting photos should get the answer from other folks.
Cheers
Keith

1:1 will give you 1 image pixel per screen pixel. anything higher than that and you're not gaining anything really. however it is useful sometimes to zoom in higher more easier and more accurate painting of the local adjustment brush

Thanks so much for your time and help. Your idea about the use of the adjustment brush with higher magnification makes sense.
I have I believe sorted this problem in the camera - seems I need to adjust by +9.
Cheers
Keith