There is some flexibility to adjust the contour levels used on maps. The Customise ~ Contour Levels menu item can be activated by right-clicking on the graphic. Select the relevant graphic (if prompted) and set new upper and lower contour limits. The number of contour levels will be reset automatically even if the number has been preset using the Customise ~ Number of Colours menu item.
The contour limits represent the end-points of the colour-bar rather than the first and last interval between the outer colours, but data values beyond these limits will still be plotted. For example, if you set the contour limits to 0.5 and -0.5, CPT will automatically divide the colour-bar into 10 colours with contours at every 0.1, but the colour-bar will be labeled from -0.4 to 0.4, and a value of greater than 0.5 will still be included in the last category.