How close can I put a 2.4GHz transmitter to an RFI-sensitive device?
It's probably best to consider a thing called link-loss i.e. how much received power you get at a distance from an antenna that is pushing out X amount of power. So, what you are trying to avoid is a power greater than -66 dBm being received from an emitter that is pushing out 1.25 mW EIRP at some distance away.

Starting with the 1.25 mW EIRP, this is about 1 dBm.

This means your link loss is 67 dB and in free-space the formula is: -

Link loss (dB) = 32.5 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.

67 dB = 32.5 dB + 67.8 dB (2.45 GHZ) + 20\$log_{10}\$(d)

Rearranging, 20\$log_{10}\$(d) = -33.3 dB or distance is 0.022 km or 22 metres.

Bear in mind that in free space, radio travels less hindered than in a factory or on earth so, in many respects, this is a cautious result. However, I know nothing about the measurement device receive antenna nor do I know if the -66 dBm was measured at the time that the WiFi was kicking out maximum power so some caution is needed.