Just so you are aware, a dish stick doubles any error of the wheel, due to the way it works; you, for example, really only have a rim that is miss-dished by 1.5mm.

Double check a few things before you touch the wheel: if it is centered in the frame (more important, unless the frame has been crashed/misaligned... But just start by checking centering), and if you didn't, use the dish stick with the tire off, and the feelers resting on the rim (regardless of if the design allows you to do it with the tire on).

When you place the stick on the rim to measure both sides, make sure you measure directly opposite wherever you take your initial measurement, or you will be taking the trueness (or lack thereof) of the wheel into account, too.