Laying out parts or sub-assemblies to determine specific spacing

This is more of a technique or "best practice" question than a specific feature question.
I've been thinking for a few days how to even ask this question and have finally settled on using a fictitious scenario to simplify things.

If your eyes glaze over reading the following question, you may want to check out the public sample document and screen shot at the bottom.

I am building a custom machine that will apply a label and heat shrink the label on a large cable every 12 inches. Each of the two steps are distinct operations that are performed as the cable proceeds through the machine.

Example scenario:

1. Cable is loaded into the machine to the point the feed rollers grab it
2. When the machine runs the feed rollers pull the cable through a set distance (steps) and stop
3. The machine first stage applies the label while simultaneously 12" further down the heat shrink operation is running
4. The machine feeds 12" of material and repeats the application/shrink operations

I don't know if that explanation adds value to my question, but I hope it does. The point is, the label application and heat shrink operation are spaced exactly 12" apart so that the label is applied, advanced 12" by the feed system and ends up in the correct location for the heat shrink operation. This is pretty basic.

In OnShape I have the following parts/assemblies (so far):

1. Various Part Studios for machine parts like frame, control cabinet, etc.
2. Assemblies for the label application operation and the heat shrink operation
3. Assembly for the entire machine

Now, the problem I'm having being new to CAD is how to position the assemblies for the label application and heat shrink operation so they have the correct spacing. I've added them to the main machine assembly, but I find that I don't know how to move them about to achieve the 12" spacing. I've run into the following challenges:

1. The sub-assemblies don't have an obvious edge or reference to use for a mating connector. I suppose I could try to pick something that is "close" to the center-line of the operation, but it wouldn't be accurate. I found myself wishing I could have a construction line on my sub-assemblies that represents the center-line of the operation (that I would use to space them apart by 12")
2. Although I plan to have the application and heat shrink sub assemblies mounted to a common steel plate, I don't yet have any holes defined because, well I don't know where to put the holes (see #1). If I had holes I could use Mate connectors to position things.

I'm stuck. I don't understand how we are supposed to precisely position assemblies (or parts, for that matter) in an assembly when we don't have a natural edge, point or face to reference? To provide another example, imagine you have a die set that are pressed together to seal something. If you wanted to position that assembly based on the center-line where the material being sealed is, how would you do that?

If you stuck with me this far, THANK YOU! I'm really looking forward to what some of you have to say. If my example and explanation don't cut it, if you don't understand what I'm trying to do, please check out the following example document:
https://cad.onshape.com/documents/3359a7d2ce8c9b7dc76a08d2/w/41a017d624d8284e3ebb5725/e/a9b07273fe2bce62bfdb3f00

Imagine that the center lines illustrated in this screen shot are what I want to use to position my parts and assemblies: