On the Canon 600D, the sensor position is typically marked on the camera body rather than listed as a measurement you need to calculate. Look on the top of the camera for the sensor-plane indicator: a small symbol that looks like a circle with a horizontal line through it. The line marks the exact plane of the sensor.

On DSLR bodies, this mark is commonly placed on the top plate near the left side, and the 600D has this indicator as well. When measuring camera-to-subject distance precisely, measure from that line, not from the front of the lens or the camera body.