Check the ROUND() function
It will round values to the specified number of decimal places.
(i.e. ROUND( 4.24, 1 ) = 4.2, while ROUND( 4.25, 1) = 4.3
To handle special requirements, (how does 4.20 rounded to the nearest 0.05 come out to be 4.5 ) you would have to write your own function to return whatever it is that you need.
Check out the MOD() function as well. You should be able to use the two to get what you want