80m just isn't needed, unless you are surrounded by say (safe side) a 60m tall forest or obstacles about that height.
Wind speed increases greatly as you gain altitude, sometimes RTH can be greatly hindered if returning into the wind (or blow away !), and going too high unnecessarily.
(How the get home or land safely in such cases is a whole other learning curve.)
I leave mine on default 30m, and assess before each flight.
In reality, I rarely use it, but good to have put thought into it before flying, just in case.
If you are flying from a hilltop for example, and flying over a surrounding valley, no trees around, say 5m up there might be ample, you could set RTH to this, as when you activate it, it will go to 5m ATOP (above take off point) or whatever RTH is set at, from waaaaayyy down below where it is right then.
RTH can be dangerous in forest, if you are under a canopy, you'd definitely want to be close to it, and perhaps disable RTH so in the event something happened it won't go straight up into the foliage.
You can click the X to cancel RTH, but often it's better to always be prepared to hit the pause button, or if quicker for you, put in sports mode to disable RTH.
DJI basic RTH is covered on pages 14 to 16 in the latest manual
https://dl.djicdn.com/downloads/mavic/20171219/Mavic Pro User Manual V2.0.pdf