there's an app called EasyMeasure - there's a free version I think and that would let you let it query your phone camera for a value.
the calibration process gives you feedback on the effective angle of view and seems to be phone model specific.
iphone 6 says 59.6 degrees.
Think Like a Spec Writer
AC4.55 through 28 / USA AC27-6010 USA
Rhino 8 Mac
MacOS 15.2