In general, you want to avoid Relations in sketches (aside from Variable Section Sweeps).
But if you are writing a Relation in Sketcher, you can use Known Dimensions in them. A Known Dimension is simply a dimension that you create in Sketcher on model geometry, not on sketch geometry like lines, arcs, circles, and splines.
For example, create a block and then create a sketch on one of the block's surfaces. When you go into Sketcer, before creating any entities, create a dimension from one side of the block to the other. That is a Known Dimension.
I avoid Relations in sketches because I've seen them give permanent yellow regen status, and they are very hard to detect unless you search for them deliberately. About the only ways I know to find them are to Edit Definition of a sketch and then go to Tools > Relations, or in Part Mode open the Relations dialog box, change the Look In filter to Feature or Section, and select a sketch.
David R. Martin II
Senior CAD Application Specialist