If the camera front-focuses consistently with several different lenses, the body is the most likely problem. If it only happens with one lens, that lens is the likely culprit.

A good check is to compare viewfinder autofocus with Live View autofocus. Live View uses a different focusing method, so if Live View shots are sharp where you expect but viewfinder AF is not, that strongly points to an AF calibration issue rather than user error.

Since you’re already at the D7000’s AF Fine Tune limit (+20) and it still isn’t enough, the camera likely needs professional calibration. If the issue is only with the Sigma, return or service the lens instead.

There is also a less-official workaround mentioned by one user: using the camera’s default fine-tune setting instead of only lens-specific tuning can effectively allow a larger correction range. That may help in some cases, but it’s a workaround, not a real fix.

So: test with multiple lenses, test in Live View, and if the problem remains across lenses, send the body in for calibration.