B = mod( ceil(step/6) - 1, 6) + 1;
The ceil(step/6) is pretty straight forward. 1 to 6 become 1, 2 to 12 become 2, ... 31 to 36 become 6, 37 to 42 become 7, 43 to 48 become 8, and so on.
mod(Value - 1, Range) + 1
is a little calculation trick. If you were to values 1 to 8 and take them mod 6, you would get 1 2 3 4 5 0 1 2 -- with a 0 in the places you want a 6. You could post-process with logical indexing or a loop to change the 0 to 6, but that is inconvenient. But look what happens if you subtract 1 before taking the mod 6: you get 0 1 2 3 4 5 0 1 . Add 1 to that and you get 1 2 3 4 5 6 1 2, exactly what you need. There are Good Reasons why this works, but it is easier to just remember the trick "subtract 1 first, add 1 after" than to work out the mathematics of it each time ;-)
