I agree with most of what has been said, although I would have no hesitation in using AirCon for the whole journey, it will take about 10 miles off your Gom/range immediately you switch it on and then that's your range. In mine at a standstill with the radio and A/C on it is using about 1amp, so negligible. As
@smokie mentioned speed is a killer, I like to cruise on ACC at about 63-65 doesn't seem to make much difference journey timewise.
Again if you can charge locally at your destination you don't need to worry about arriving quite low but mid journey stops I like to be stopping at about 150-180 miles, just my preference, but for a journey of 300 miles I would push it to 180-200 to allow for adding enough to complete the journey and I would probably top up to about 85-90%, but no more, again just my preference.
I'm off to Southampton in a couple of days from Leeds and am planning on stopping at Banbury which is loaded with chargers galore of all different colours but is only 150 miles leaving 100 to go.
I did quite fancy stopping at the new Fastned chargers at Oxford in the park and ride at about 180 miles but there doesn't appear to be any facilities for relaxing whilst charging.