How do you define the 'slot'?

[Bluefoot]

I've worked ASA SP for a few years, and this season will be my first working FP.

What is the correct positioning for the slot? I asked a local UIC, and all he told me was that for a right-handed batter, the umpire's right eye should line up, horizontally, with the inside (or left, from our view) edge of the strike zone, ideally. Can anyone please give me a more specific description?

Thanks.