Thank you all for your explanations! Obviously my calculations had
been setup too quickly ...

>   13*7227*65536 / 7200 = 855162
>
> The textheight is then
>
>   39*855162 + 10*65536 = 34006678
>
> In points this is
>
>   34006678/65536 = 518.90073

13*7227*65536 / 7200 = 855162.88
                             ^^^
and then 39*855162 + 10*65536 = 34006712.32
                                     ^^^^^^
34006712.32 / 65536 = 518.90125

Ahhrrgg. I forgot to do integer calculations ;-(

Best regards,
Ulrich Dirr