Sigfox RF module TD1208R: design a 50 ohm microstrip line on a PCB (Proteus ARES)
You don't have to care much about impedance if the length of the trace is significantly shorter (eg. by a factor of 10) than the wavelength. 868MHz is 
 about 34,6cm long, so if your trace is shorter than 3cm - any width should work.