Convert coordinates from picture to textpos [duplicate]

Question

I am trying to convert some code from using the picture environment to textpos for a titlepage but I have run into problems getting the text to align.The coordinates are given in points.

the original text is placed with

\AddToShipoutPicture*{\put(50,583.5){\fontsize{20 pt}{22pt}\tyk \OPGAVE}}

where the t-coordinate is given form the bottom of a (A4) page presumably with the lower left corner at the coordinates.

And the new text:

\begin{textblock}{200}[0,1](50,258.5)
     \noindent\fontsize{20 pt}{22 pt} \tyk \OPGAVE 
\end{textblock}

where the y coordinates is given from the top of the page. I calculated the new y coordinates by y_new = 842-583.5=258.5, since the vertical dimension of a A4 page is 842 points.

The red text is the original.

Any assistance would be greatly appreciated!