Unless your cartridge has a huge assembly error that you want to compensate for, set your tonearm's height so that the upper surface of the cartridge is horizontal when the stylus is resting on the LP with correctly applied downforce. There's a cheap tool you can buy on
Amazon or
Aliexpress that simplifies the procedure a bit.
If you have a carpenter's laser level, you can also use it to ensure the cartridge's top surface is exactly horizontal.