Longpress START and longpress LAP would be both useful and can coexist.
On a trackfield you want to have the smallest perimeter as possible cause you want your lap to be as near as where you started (not 25 meter away from your departure). The opposite is valid for larger area. Roger c has already proposed the same while ice skating in circle surface.
I'm not sure if I understand you correctly
, but Lap_Time and Lap_Point might be more accurate if calculated in this way.
But for the lap time you should expect to be left out of the area determined by the desired diameter.
You could have a first acoustic signal at point A (with a pop-up indicating you have reached the LapGPSPoint), and the calculated Lap_Time to point B.