What is the recommended distance between fly control units?

Is there a recommended distance between fly units? I am trying to convince my manager that we need to have a second fly unit in an area. 

if you still have flies then 1 is not enough

generally speaking, 1 per 40 x 40 room is usually sufficient, anymore square footage than that a 2nd unit is wise

You may also need to consider having more than one if multiple points of entry exist.  One light & glueboard device may be sufficient for 1-2 loading bay doors, but each door after that will drive the need for additional control.

How many are needed will depend a lot on your geography/climate/season and native pest load, as well as the physical layout and construction of your facility.  

We have a fly unit near each roller shutter door. 

Sure the cost is a little extra but it provides extra reassurance that Pest Control is treated properly and reduces the risk of flies bypassing one unprotected door.

We have on at each exterior door. We had to move one during an audit as it was actually 18' from the feed of one of our machines (just put new screws in 2 feet further away!) Power sources are a potential issue with this one. You might have to create new power runs or have them run for you. Our in-house electrician takes care of ours.

In a pharma company, we had one on every just about every wall. I'd say it was overkill xD