Fiddling around is also my preferred method when I want to understand how something works.
But if you are looking for a quick information you could search the forum or the KM Wiki:
A forum search for “decimal” (one of the words in your topic title) gives you a very good hit on position #2.
A KM Wiki search gives you a hit on position #1 that contains a link to the above mentioned forum thread. Search result #4 gives you the complete explanation of the Format token.
Or open the “Help” in the Gear 
menu of the Calculation action, which leads to the same Wiki article as #1 in the search. See this very nice Wiki article for how to get immediate help from within the KM Editor.