Best practice of SC on Server Side Include pages

I am working on a modular web app and using Server Side Include (SSI).

Could someone share the best practice of using Server Connect (SC) on SSI pages? I am running into trouble when I am loading the SSI pages across various head pages. What is the best practice?

Is it:

• Option A:
  Adding the SC to the main page and the SSI loads it from there?
  What would be the path to the SC? Would it be {{SC}} or {{someID.SC}}?

• Option B:
  Adding the SC to the SSI page and load it from there.
  What would be the path to the Service Connect? Would it be {{SC}} or {{someID.SC}}?

• Option C: …?

Also some clarification/documentation on how to indicate hierarchy between main page and SSI pages would be welcome.
For example, how is the main page identified in the code?

With these:
<base href="..."> and meta name="ac:base" content="...">

Or on the SSI page with this:
<!-- Wappler include head-page='main.php' [...] >

Or is there some documentation I’ve missed?