Choosing the right Brown-out level
Brown-out detection and frequency are two independent things. Another core aspect to consider is the operating voltage. Your operating voltage determines possible frequencies (higher voltage allows higher frequency, but nothing stops you from using max voltage with the lower freq of course).
BOD is nothing more but a voltage at which the MCU will stay in reset and not run the program (and eat a few orders of magnitude less power).
You can have an ATMega328P (arduino uno) run at 5V with 1MHz or 8MHz, and you can have BOD at 1.8V, 2.7V or 4.3V (or completely off, then it may go unstable near its lowest operating voltage spec)
Alternatively, you can run the same MCU from 3.3V and you, obviously, can have only 1.8V or 2.7V BOD; If you set BOD to 4.3V with Vcc=3.3V, your MCU will stay in forever reset until you apply &gt;4.3V to it. If it's soldered onto its own PCB, you'll have hard time returning it back to life, possibly will have to solder it out (you could damage 3.3V regulator powering the MCU if you apply 5V there obviously).
Frequency doesn't have much to do with it, it's only that if you need higher frequency, you may need to go for max Vcc, so you can set BOD to higher level too. Your BOD options are determined by Vcc. And Vcc MAY or MAY NOT depend on desired frequency.
Of course, this logic applies to your MCU as well, nothing new about it. Yet again, if you're powering the MCU from the voltage regulator, you probably don't need BOD at all since the Vcc will always be the same (unless the regulator input voltage falls, but it's better not to go out of input voltage spec of the regulator or make sure it's safe that way).