I’d rather go for something more targeted to programming. DT doesn’t teach you anything there, it just allies you to apply your knowledge.
And personally I’d not go for Applescript. That’s just an outdated, unmaintained and under-specified language. JavaScript is more useful generally nowadays. But there are other opinions on that out there.
I am going to jump in here in support of @BLUEFROG comments. I used the Johnny Decimal system for several years myself. From the website, it sounds very logical and a good method. However, as @BLUEFROG suggests, it is worth stepping back and thinking about what it is you are trying to achieve.
In my case, I dived straight in and soon ended up with a huge hierarchy which became impossible to manage and I had no hope of remembering what “20.12” was. Constantly looking up against the index rather defeated the aim of an efficient, quick method to file things. One of the core ideas is that the numbers are used everywhere (DT, Mail, Other apps, Finder) so that everything pertaining to a particular ‘topic’ can be found anywhere.
I then did the more sensible thing of standing back, thinking hard, and designing a hierarchy that fitted me and made sense (to me). This took several iterations using mindmaps. Some initial iterations fell apart as I started to load them with real documents (the problem usually was a document that could be put in several places). As I think they say on the website, it is important to design your index/hierarchy first and then put items into it. For me, the index/hierarchy (apart from the project section) was essentially static and only changed when something new definitely didn’t fit with an existing section (e.g. starting a new role).
FYI - after several years of use, it morphed into a system of letters and numbers. For example PRJ.01 (for a project), Fin.02 (for finance-pay slips) etc. as it was more flexible and easier for me to remember without having to refer to the index all the time. What I was trying to achieve? - an easy to remember system.
I think, what you are saying is similar to what I was describing in this comment?
I completely agree with you, without Mindmanager and mindmaps it would be almost impossible to come up with a correct system. After all you are setting decimals on it. Unless you want to end up spending time renaming it, better come up with a plan and structure.
I have a feeling me saying I am new to this came across as I am ignorant or whatever. As I said, I have gave this some thought and planned it out.
I have no doubts I will have to iterate it when the rubber meets the road, but I have not just woke up one day out of the blue and said ok, these decimals look so cool, I will spend next month or so creating folders for no reason. 
And yes every system will has it’s pros and cons. Knowing myself and my fishlike memory, I need structure and S.O.P.s in my life.
Anyways, I’ll give it a go. If it fails I can always go back to PARA. Worst scenario is a failed experiment and lesson learnt.
I don’t understand the purpose of your numbering
Personally I prefer tags, for example PRJ.aaaaaaaaa, FIN.PaySlips
- also an easy to remember system🙂
I agree and I am looking to switch (which goes to show organisation systems are vey much a personal thing). Tags are much easier to add and I can have a document in two places 