I'm not sure what you know and don't know, but I'll put some input in anyway. ;o)
Apologies too, if I've misunderstood your problem, online comms is often full of mismatching of ideads etc.
Okay, so ... the supply pinch roller and its associated nut: the turning of the nut determines where the white guide (and roller) will 'sit'. To see the guide in action, you'll need a 'cut away' shell with tape inside, or a another test cassette that is transparent. This way you can see if the tape edges are catching the guide. During PLAY I simply lifted the guide up/down with my finger nail (not very technical I know!) and from which I could determine when the guide and tape 'cleared' each other.
BTW, and unless I've got this wrong, you may need to remove that plastic cassette loading tray - take care since the 4 screws are small; they can fly off at 100mph (a magnetic screw driver helps!) And there's a set of black plastic cassette shell 'guides' in there. Make note of which way they are seated before removing this.