You want to place your extender about halfway between your main router and the area where you want to boost the signal.
Too far from the router and the signal to the extender will be weak, but too far from the target area and the signal there will be weak.
If you can’t get it close to the middle, you may have to play with the location a bit to find the sweet spot. If in doubt, start closer to the dead spot and then move it towards the router if you don’t see the expected results.