Calculating RF attenuation through plastic container
There are so many more factors that will have an impact on transmission distance than any supposed attenuation through the plastic enclosure that you should adopt a different approach. One practical approach is to use a few of your units and range them apart in various orientations and environments and check if communications is even possible. Then change the distances and/or orientations and try again. 

With some careful iterative testing like this you can come up with some real numbers to tell your customers. 

Even better is if you can be running some type of test protocol over the RF link during the testing that includes an error correction algorithm. You can use the instances of amount of error correction happening to know when the units are at marginal distances for reliable communications. If the packets were transmitting continuously with results on a display you could simply walk around with the unit.