In theory below 4. I say in theory because there is no objective evidence establishing Kp index having any effect on low altitude, operator line of sight drone operations. It has never been an issue with radio controlled aircraft on 53MHz, 72MXz, or 2.4MHz model aircraft systems. Having flown RC for well over 45 years I can assure that “radio failures” I’ve experienced during that time were usually my fault for one reason or another. Shoot, I wasn’t even aware of Kp index until a couple years ago.
Our multirotors use common design 2.4MHz RC transmitters and receivers that function the same as RC fixed and rotary wing systems. Perhaps of equal value is that we don’t need GPS at all to operate our aircraft safely, we only have to be more attentive to direction and position control and as long as we can see it we can control it.
Most of the consumer drone systems default to a “Manual- Atti-Stabilized“ type mode when GPS (in and of itself) fails, leaving the aircraft fully controllable afterwards. The only “danger”, if we wanted to call it that, is the aircraft will drift in the direction the wind is blowing. If left in a hover the altitude will hold but the lateral position will “drift” off course in the direction and speed of the wind. In lateral flight we have to input a little angular correction to offset for wind. Had we been flying an automated flight plan the aircraft could no longer follow it if GPS failed, we would have to assume manual directional control.
With all that you might have concluded I don’t put much stock in the Kp index thing. Personally, I don’t bother looking it up. We flew every day and night for many hours at a time in Iraq, Afghanistan, at sea, and other places at distances many miles further than multirotors can be flown with no concern for Kp index, ever. Sure, GPS failures happened from time to time but the solution was the same then as it is now; fly the aircraft manually. Sure, it was a bit of work flying something back from 30-70-150 miles out with no position fix and only the camera and compass heading to use for location and direction reference but people trained for that and dealt with it. Much, much easier with an aircraft we can see when flying them.