How detailed should a high level process flow diagram be?  Well the answer is....it's a judgement call.  This isn't a bad thing.  It means you can get it wrong. Process flows are the most understandable when you start as a very high level and then slowly broken down by expanding on sub-processes. 

You can compare creating process flows as writing a 10 or 20 page term paper.  You don't just sit down and write 10 or 20 pages.  You start with a basic high level outline ensuring that you hit on all of the relevant pieces.  The high-level process flow is the same.  It's your outline.  But all of the relevant activity should be there (just without the detail). Be sure not to leave out major pieces.

Then you detail out your outline further adding sub-bullet points or even a sentence or two about each bullet point.  This is like detailing out your sub-processes within the process flow.

Process flows can have as many levels as you need.  So your first high level process flow may have sub-process that are then detailed out.  Those sub-process can also have sub-processes within them.

Hopefully, this helps and eases your concerns.