Postmarks: Particular Requirements for "Socked on the Nose"?

Opinion
the one that suits me, is the ability to read and understand the town, date, and important info on the hammer face
Early postage stamp formats  / sizes,  preclude to getting a full face S.O.N.

Collect what you fancy, and over time, you'll know what parameters suit yourself.

Welcome to the discipline,  it's a fascinating adjunct.

 Warning: It is very common to get egg on one's face, when interpreting postmarks, or part thereof.
wear it well, for when you, on occasions crack the correct postmark, there is a great feeling of joy.

...and a not so good S.O.N.  but still very collectable.

Maltese Cross Numeral 11, with cross at top
Exist : Numerals 1 to 12  (but NOT numeral 3)
This type in use at various London Post Offices 1843-1844

Just to clarify, the Maltese Cross #3 cancel does exist, however, it doesn't have the additional small cross at top (the other numbers do, like the #11 shown by rod222).

A lot of the best early postmarks were CTO, by order, complaisance as the French say. But that is how many of the early colonial postmarks were preserved.

As you dive into Postmarks,  I would suggest you will experience devolution, 
and begin to collect other forms than "S.O.N"

You will maybe become  interested in EKU  and LKU
Earliest known use , and latest known use   (Other countries may use differing acronyms)


With Australian Postmarks, single penny stamps with good postmarks can regularly attract $5 - $50 each.

Or, you may hone your discipline to just one country


Belgian "Pizza Wheels"  for example



Or Belgian Railway Parcel cancellations, I have well over 1000, and yet hardly scratched the surface
You can track rail lines throughout Belgium