Imagine you're abbreviating a title that has a kn-word in it (e.g. Should Know).
What is the preferred way? Is it SK or SN?
Abbreviations are usually based on the first letters of the words they represent, and I would choose "SK" in this case (additionally "SN" is a well-known abbreviation for "Serial Number" across many different industries, so it's probably a good one to avoid when possible as well).
Consider also the word "Herbs," a word for which some people don't pronounce the letter "H" -- regardless of how this word is pronounced, an abbreviation would normally include the letter "H" to represent it (using the letter "E" would be confusing).
Kn starting a word would become K, by convention. For instance, JSYK is common shorthand for Just so you know.
AFAIK, know is often abbreviated to K.
More generally, both phonetic and spelling-based abbreviations exist.
Abbreviations could be with the pronunciation of the word, for example, 'n' for 'and', and 'k' for 'okay'. Other such abbreviations can be seen here.
It really depends where you are. If you are texting, then maybe 'N', if you are putting it as a message, probably either, if you are putting it as the name of something, probably 'K'.