I think you're on the right track, but I think capturing a trending pattern needs to happen first. And then if an area is observed to have the same high noise level consistently, then look into placing another AP near that area, or rearranging the currently deployed APs to cover that area better. Some customer locations where -88 dbm is the average noise have had VOIP working perfectly over the wireless. It's something that you'll want to test and observe prior to making it your magic number.