Below 1B1 is 1A1 (675)
Below 1A1 is 191 (650)

There is something to be said for the efficiency increase at lower speeds.
I keep it in mind as the efficiency  tradeoff for speed becomes moot at some difficulty level.

Above 251 is 261

ignore the last digit

 950 MHz would be 25 (251)
 975 MHz would be 26 (261)
1000MHz would be 27 (271)
Increase by one

It's hexadecimal so A comes after 9 (instead of 10)


TANGENT alert!
To calculate the value in the last column
Again ignore the last digit, multiply by 2 and add 1
(determined experimentally)

ex:
using 825MHz because the hex and decimal LOOK the same and the math works.
825.0 MHz    201        412

201, drop the 1
20 x 2 = 40
40 + 1 = 41
add a 2 to the end for last column
412 is the same as 201

Think of it as 20 divided by 1
or
40 divided by 2
(plus 1 for the div by 2 instance)

Most should have a calculator with hex mode, try it on a wierd one with a letter!