It is still running too fast
The code link in post #17 shows how you can set the fuses, post #19 mentions the CKDIV8 fuse and also that you can set the prescaler at runtime regardless of what the CKDIV8 fuse happens to be.
The timer clock will be a known speed- you know the source clock, the cpu clock prescaler either via CKDIV8 fuse (/1 or /8) or set on your own via CLKPR, and the timer prescaler (/64 it appears you want). Your OCR1A calculation also has to be correct, which by your original post will be 2x faster than it should be (an extra /2 included because you were reading a toggle freq formula).
Without much work, you can create a simple test by toggling an led with the help of the timer so it will be easy to tell if the timer is running as expected. Not much fun chasing unknown clock speeds, so may as well get desired==actual before you get too deep into code that has magic values inserted to 'make it work'.
A simple modification of previously linked code-
https://godbolt.org/z/b5o8Eq678which should blink an led at 1Hz- it either does or does not. If not, something is wrong. Create your own test as you see fit to get the timer running as expected.