Thorens TD124 “Mk1”/Mk2 plinth

I use Decca cartridges which incorporate relatively large magnets with my MKI TD124. I have no issue with the original iron sub platter EXCEPT this: I make tracking weight adjustment for these cartridges with an LP vinyl placed on the platter mat. The mat incorporates metal or other material interwoven (Auditorium 23), however the magnetic pull of the platter still affects downforce on the cartridge if downforce reading is taken with only the mat in place. Placement of an LP under the stylus to then check tracking weight, for my system, is the only way to obtain a more or less accurate readout. YMMV.