To my mind CARVER system is good at prioritizing high-level goals or projects, but too complex for prioritizing small tasks, taking sometimes more time to prioritize than to complete the task
For small tasks I'd prefer using Covey's Urgent/Important Matrix, which works perfectly in Allen's GTD as well. In order to be used for prioritization Urgent/Import Matrix can be given following scores, for example:
4 - urgent & important
3 - urgent & not important
2 - not urgent & important
1 - not urgent & not important