I have heard that the tool is difficult to use and/or may require sending the cartridge to Wally
The Wally zenith tool is easy to use IF you send the cart in for microscopy analysis which precisely measures zenith error as well as providing a custom shim to optimize SRA/VTA at the headshell, which is what I did.
Otherwise you are correcting by ear which I imagine isn't easy, hence Bill's comment that AnalogMagik is easier.
I have not used AnalogMagik so I am not advocating one method over the other.