I have a process-monitoring system which is sensitive to RFI, specifically in the 412-419MHz range. It has not been tested for sensitivity to 2.4GHz, but I've measured a WiFi signal level of -66dBm at the device in question while it was operating with no issues. The transmitter I'm concerned about has a max output of 1.25mW EIRP at 2.42-2.45GHz.
How do I determine how close I can let the transmitter get to the RFI-sensitive device without exceeding the existing signal level in that location?
I've tried reading into some wireless range and power loss calculations, but this isn't my strong suit and I'm not getting very far. I'm not necessary looking for just a distance, but the process to determine it.
Edit: Considering the unknowns in this situation, I understand that it's not feasible to get a precise value for distance for this particular device. However, the discussion below gives me a bit of a ballpark to go on and I expect that I'll be heading towards some testing to confirm. If any useful information arises at that time, I can incorporate it into this question, but I'll consider it closed for the time being.