Se up your sound system with all its ancillary equipment. Check everything is working in the normal way. Leave any delay unit settings as they are for now.
Measure the distance from your main loudspeakers to the delay loudspeakers. This distance is part of the calculation you need to set the correct time.
Make sure you know the current temperature of the venue. This affects the speed of sound and therefore your system setup. As the formula required is complex, use an online calculator to find out the current speed of sound. With an online calculator such as the one found at HyperPhysics website (see "Resources"), the current temperature is all that is required to find the speed.
Divide the distance between speakers by the current speed of sound. For instance, if your speakers are 50 feet apart and the speed of sound is 1126 feet per second, then the delay time is 44.6 ms. Enter this number into the delay line controller.
The system should now be set with no delay between speakers and therefore no echo. The temperature should be monitored in case of sudden changes --- a hot crowd could change it quite quickly. Many delay units now have temperature gauges built in so they can automatically adjust the delay time.