Although I think I’ve long had this working good, I did some more tests just to verify.
Each cycle is about 15.8us, 2us per bit, 8 bits, this sounds fine. This does confirm, however, what I constantly see about the data rate being actually a little bit higher than 500kbps for the raw amiga MFM. Some other numbers from today 3.9 (for 4us), 5.88(for a 6us), and 7.64(for a 8us). That 8us one is always low! I’ve seen it hover around 7.5 before!
The PC is reacting quickly, and total worst case I saw today was about 9us for the whole cycle.