Well i found this
Td = T - ((100 - RH)/5.)
where Td is dew point temperature (in degrees Celsius), T is observed temperature (in degrees Celsius), and RH is relative humidity (in percent). Apparently this relationship is fairly accurate for relative humidity values above 50%.
Tho the more accurate calculation is too complicated for me again and needs conversion to kelvin
This here also seems to be a good (and easier to understand) paper even with different accuracy ranges (see the table 1) i would probably like to use -20C to 50C https://web.archive.org/web/20200212215746im_/https://www.vaisala.com/en/system/files?file=documents/Humidity_Conversion_Formulas_B210973EN.pdf