Take a look at figure 5.9 page 61.

The right hand drawing, side view, the 2mm dimension is shown. The lower corner of the threaded boss is 2mm below the bottom of the bush tube.

As has been said the important things are to place the threaded boss and the bush tubes correctly, you can think of the wishbone tubes as "connectors", in place to hold the threaded bush and bush tubes in the correct orientaion.

It's not all guess work you know

Cheers

Chris