datum set on mounting surface of linear motion guide

[bxbzq]

Hi,
Our linear rail assembly scheme is like attached. Now I need to figure out proper datum set. From assembly process standpoint, the shoulder should be specified as secondary datum. But the whole assembly (not shown here) is almost symmetrical about the center line derived from two lines of threaded holes which are used to fix the rails, while the shoulder is one of the non-symmetrical features. So naturally assigning the center line as secondary datum is straightforward and creates no confusion. I would like to hear opinions from others.
If the description does not give sufficient info, I'll try to post the drawing.

 

[CheckerHater]

Threaded hole by itself is not the best choice for datum feature.
Deriving imaginary center plane from the whole bunch of threaded holes can make many people uncomfortable.
As a matter of personal opinion I would go with the shoulder. It's functional physical feature that's easy to deal with from machining and QC perspectives as well.

 

[fsincox]

The shoulder is by design the secondary datum in your example, that is it's function, dimension it basic any way you want but the shoulder is the secondary datum reference surface. I have worked wit these before and did it the same way.
Frank