I developed a coded language when I was a kid, which I still use sometimes. I have looked at Ricky's notes but all I have is some theories based on my experience with my own language. Hopefully that helps.
My own language is letter-for-letter encryption, which makes it a lot easier to memorize the key (I was 8 years old when I invented mine). I suspect that is what Ricky did since I noticed that some individual characters were corrected.
Also, after almost 40 years of using my own language, I have developed "shorthand for the shorthand", so some of my words are abbreviations or shorthand for actual words - which makes letter-for-letter decryption tricky. When things get complicated, I insert a character I know does not belong in my key to indicate the beginning and end of some shorthand alteration. I suspect Ricky might have used parenthesis.
My key duplicates some characters (i.e. they mean something else depending on how they are used). I am wondering if Ricky's numbers are not always numbers. One piece that struck me, was the "99.84S2..." sentence because in my own system I don't use the number zero. So I thought, wouldn't it be interesting if Ricky thought the same way, and a period is shorthand for a zero in a telephone number?
I do not write things down in code just for the fun of it. Although it is fun trying to decipher code. I use it to write down passwords, bank account numbers, etc. that I don't want anyone to know. In other words, very precise pieces of information. The problem is, for all we know, Ricky could have been an aspiring artist and these could be his lyrics that he thought were worth millions. This is a lot of writing for one codified note. My guess is that these are several pieces of information that may not be necessarily related, such as a list of contacts or a log/diary of some sort. The note on the FBI site looks like it was all written down in one sitting. Therefore, there is a high likelihood that these notes were transferred from somewhere else (or Ricky's head) as a way to safeguard something for himself and nobody else (i.e. the law). Therefore, the diary/log idea is less likely than a list of contacts or places.
I would love to hear how this story ends.