How to measure a film thickness using XRR on a D8 Brucker?

I am having a film deposited on Si. I know the thickness is something between 50 and 120 nm (SEM cross section plus step methods). I am trying to validate this values bx doing XRR measurements. I am having a D8 Brucker diffractommeter with a mirror on tube side. Both slits are 0.2mm. I am following the procedure of Scott Speakman (see attached file). However I am not getting any nice interferences fringes (as previously on some different machines) and therefore I am having some related questions. Everything is going ok until step IV - page 7 of 13- (allign the total external reflected beam). There is written to set 2theta drive at 0.4 degree in order to perform a correct alignement. Then it follows the step V reffering to the allignement of reflected interference fringe. my problems start here as I dont have any interference fringe in the 2theta omega scan. My question is now : the 2 theta drive should be maintained at 0.2 degree as in previous step (that seems logic to me in order to have an incident angle) during the XRR scans or should be fixed again at 0 degree? Can anyone provide please an alternate (maybe more simplified ) procedure to determine the thickness using XRR?. Thank you