Rise over run... think of a right angled triangle... especially if there is any terrain or obstructions along the way. If you have a clear path between the remote and the aircraft, that is your best setup for distance.
Me personally, I tend to go to max height allowed if I am looking to fly max distance.
You can minimize obstructions by taking off from an elevated point such as a hilltop. And a parabolic antenna booster can add some distance cheaply.
Also need to account for any wind...
Generally speaking, the greater the altitude the stronger the wind. You can't perceive this effect from the ground. So going to the maximum height of 400ft (120m) unnecessarily can work against extended flight time.
Best practice is to fly against the wind going away and then let the wind assist you on the way home. A cross wind adversely affects you in both directions.
Greatest distance will be on the truly windless days. Lots of YouTube videos demonstrate the above
Consider also, that the wind forecast from UAV or other sources is based on one location, as your distance increases, the wind can vary considerably. I am very particular about the wind values if I intended to stretch the distance. And as others have said, into the wind on the first half of your flight, back with the wind behind you... Compare the gusts to the persistent wind speed... its those gusts that can lead to all sorts of nasty things, including YAW errors and ATTI mode.
As for KP.... this is the definition in UAV Forecast.. however, I have heard dissenting opinions on how risky it truly is....
The Kp index measures geomagnetic disruption caused by solar activity, on a scale from 0 (calm) to 9 (major storm). The higher the Kp index, the more likely you are to have problems. Kp's of 1-4 are completely safe. At a Kp of 5, you'll have problems perhaps 1% of the time. At a Kp of 6, you'll lose an average of 5% of your locked satellites; at 7, you'll lose roughly one third.
Solar activity interferes with GPS signals in two ways, both due to disruptions in the ionosphere.
(1) It decreases the signal-to-noise ratio and affects carrier frequency, causing the receiver to lose lock on some satellites. Instead of 9 satellites, you might lock only 6, or the number might fluctuate from second to second.
(2) It changes the propagation delay through the ionosphere, making GPS positioning inaccurate even if the receiver has all satellites locked.
Even during a major storm, the extent to which you see these problems will depend on many other factors, including your latitude (worse at high latitudes) and the time of day (worse at night). Sometimes everything will be fine. At other times you'll lose lock completely, or appear to have a lock but the position will actually be wrong by hundreds of feet.
The storms can also interfere with radio control signals, or with the onboard electronics.
So in general it's better to play it safe during solar storms at Kp of 6, even though most of the time you won't notice any issues. At 7 or higher, you're better off sitting inside and flying a simulator. Happy flying!
For current and forecast Kp conditions, as well as the weather, see http://www.UAVForecast.com.
I am one of those that flew autonomous missions using home made drones during a solar storm in 2016.
I flew because I had no idea until after the fact I started hearing about it on the forums.
At no time did I have any issues with a Kp of 7 during that storm.