Exactly. I worked it out separately, and that's the correct formula.
So the notional resolution-2 decoy could have a GPS of 2.052, and a detection range (EM 11) of 22,572 km. Yes, in theory it can detect anything with an EM 11 sensor before it is detected. This is obviously bad in a decoy. Now, if we made the sensor resolution 100, it could be picked up by our notional sensor at 1,128,600 km, and would itself have a range of 1,838,477 km. The detection range is still above the intercept range, but not as much as before. I can't recall what the resolution cap is. If it's 500, then you have an EM intercept range of 5,643,000 km and the sensor itself would have a range of 4,110,960 km. (The intercept range scales linearly with resolution, the active range with the square root of resolution.)
I plan to test decoys in one of my games before too long, to see how the AI behaves around them.