Specifically need advice about how to buy replacement for the rear cassette on a Scattante R 670 road bike (Shimano CS-5600 10 speed 12-25T ) to spin up hills with less effort. What do I look for so I purchase the proper fit. I know I want a number greater than 28, but should I keep the first number (12) the same?? All new for me..any help would be appreciated.