Thank you for your post.
The Ceiling function will take any decimal value and rounds it up to the next integer. Since you want to round up to the next .5 value, multiply the initial value by 2, take the Ceiling of that value, and then divide by 2. For instance, if your Number field is named "Number", the calculation would be:
Ceiling ( 2 * Number ) / 2
Using your example of 1.2 for Number::
Ceiling ( 2 * 1.2 ) / 2
Ceiling ((2.4) / 2
Let me know if you need additional clarification.