When I go on a SOTA activation, I always hike up until I reach the very summit, marked by either a trig point, mountaneering mailbox, just a cairn or by simply being the very highest point within the summit GPS coordinates. However, I’ve sometimes needed to move away from the very summit and the way I’ve used to know if I still was inside the AZ was by setting to zero the relative height measured by my CASIO wristwatch altimeter while at the very summit and then hiking down while reading that relative height measurement. As long as I read less than 25 (to be precise, more than -25), I know I’m in the AZ.
The altimeter of my CASIO watch is based on barometric pressure but the baromatric pressure variations in the time window between setting the relative measurement to zero and hiking 25m down is so small that I’m sure the reading remains accurate enough.
However, to be absolutely sure, it will always be safer setting up at -18 than at -25 with respect to the very summit.
73,
Guru