FCB means "Form Constant Byte" and holds 8 bits FDB means "Form Double Byte" and holds 16 bits.

LD000 FDB $4A3F stores the value $4A3F at location D000.

Yes there is a lot of math taking place in the code. Scaling variables, looking up 3D tables etc. If you are trying to re-invent the wheel, I would start by getting a high level understanding of how the code flows. What the interupts are, and also the order things are done in. Then you will need to understand all the various subsystems and how they work. Then you could work on how the tables hold calibration data for the tune. I am an experienced programmer who has managed a very large project, and was responsible for providing time estimates for bids on custom programming jobs. My estimate would be at least a year of full time work to understand what is happening in the code. Good luck, and ask lots of questions