You need an overhang/cartridge alignment protractor that aligns the cartridge for proper overhang. It connects to your spindle, and move the cart back or forward so the cart is aligned with the protractor grid at two given points marked on the protractor, not skewed from the grid. To achieve this, you also do not want the cartridge skewed at the head shell mount either. Moving the cart back or forward will get the alignment correct at those two points. If they are not, your cartridge will be out of alignment.
You might find one locally, or as example, can get one from Hudson HIFI, or similar.
There are much more complex protractors, but I would at least get a basic one for about $10 to get started.