You dont have to hit it precise. After just about hitting zero, use the side arrows below to adjust sideways., until you are satisfied with result.
If was is mounted with arm almost at zero with straight wheels and you use almost full degree of your was, then changing was probably won’t help.
Depends on tractor model and how you are going to mount it .
Spring loaded is better but not essential.(Try to add a spring on your was arm on which ever was you have now, to test if that is your problem)
Lots of models here! Steering angle sensors - #62 by Reed
But again important is how you use your was, and if old tractor how much play in the whole steering system?