Here are some fundamentals of how Repeaters perform.  The REx20 Repeaters and RE524X have two large, unobstructed antennas, and have industry leading RF circuitry.  This means:

• The Repeater can hear better than most any control panel.  This is because most control panels antennas are stuffed behind a bunch of circuit boards, close to noisy things like LCD displays.
• The Repeater can talk better than most any sensor.  This is because virtually all sensors have only one antenna, so they are subject to dead spots in signals.  Plus, they usually operate at a higher transmit power than virtually all sensors.
• The result of the two phenomena above is that you can often mount a Repeater just about ANYWHERE in a house and it will improve the situation.  But if you have to mount it for best performance, mount the Repeater nearer to the panel than to the sensors you are worried about.  This is because the panels (particularly the self-contained variety) are usually the weak link in the system, and need to be wirelessly-yelled-at.
• To troubleshoot or optimize the system, use the green LED to verify that the Repeater is hearing every sensor you are worried about the panel not hearing.
• It is good practice to enroll the Repeater itself into the panel as a monitored zone.  In addition to tamper and low backup battery monitoring, it ensures that the wireless connection between the Repeater and the control panel is always operating correctly.