Vespa px whit parmakit ignition syste star in REVERSE

Hi i have a big problem with my vespa in the past few months i tried to set the timing in my vespa with the parma kit system and it has been a pain to set it. i ordered online the timing disc and the piston stopper, and i have been searching information about how to work with the timing disc. i already know where is the fire ignition timing point for the DR (19) but i don't know what to do with all this information that i have, i don't know how to use it to fix the timing on the vespa.  the vespa starts but it starts on reverse. i stiill haven't used the timing disc because it hasn't arrived but as i said before i don't know how to use all the information that i have to fix the timing of the vespa once i have all the parts ( the disc and the piston stopper). if you can bring some light to this, i have been reading your website but i haven't found how to use the disc and i haven't found information about how to solve my problem. if you can help me or give me additional information i would thank you