Hi Pavel,
I know what you mean. However,
* the original question did not mention this specific situation with a singke supply, and
* the parameter Voff is specified as mentioned before only for double supply.
* In case of single supply I think, you cannot "adjust" the specification.
The reason is as follows: The dc output will be never exactly at Vcc/2 and it is by far not correct to allocate this
deviation to any unsymmetry of the opamp input (that - in fact - is the cause of Voff). In contrast, the output deviation
with respect to Vcc/2 will mostly be caused by tolerances of the supply voltage and of the resistive voltage divider.