When making adjustments with the Color Balance tool you can enlarge the panel so that it is a little easier to make ultra-fine adjustments. You can take the Color Balance tool out from the Color tool tab by clicking on its title bar and dragging it to the Viewer.
If you use the Color Balance tool frequently, consider adding a dedicated tool for color grading. If you triplicate the tool in the tool tab (right-clicking in a tool tab and selecting Add tool -> Color Balance), you can display individual panels for Highlights, Mid-tones, and Shadow. With separate panels for each, you can avoid moving between tabs for the larger color wheels.