How to create a centerline at a certain distance from another centerline ?

It is an issue with how Creo uses references for the intent manager to create dimensions in the sketch. I am not aware of a way to select a sketched centerline as a sketch reference within the sketcher. This type of scenario is addressed through the use of sketcher constraints and sketcher construction entities.

Overview

Construction geometries are signified by dotted, magenta entities within Sketcher. Construction geometries are important because you can use them to constrain your sketch easily. With construction geometries, you can control design intent, simplify dimension schemes, and simplify sketches. To create new construction geometry in Sketcher, you can toggle Construction Mode on, and then you can use any sketch tool available to sketch new geometry. However, the resulting geometry is created as construction geometry rather than solid geometry. Once done, you can then toggle off Construction Mode and resume sketching solid geometry using the same sketch tools. You can dimension and constrain construction geometry in the same manner as regular solid geometry. Construction geometries do not add entities to the final sketch, and therefore they do not display in the final Sketch feature. You can convert any solid geometry to construction geometry and vice versa. To do so, select the geometry entity you wish to convert and click Toggle Construction from the mini toolbar.