Ah - I missed the “x 2” bit at the end of the maths.
83° FOV is the diagonal angle
Ground diagonal = drone height x tan(FOV ° / 2) x 2
=120 x tan(41.5°) x 2 = 212m
4:3 sensor means that’s 169m wide x 127 high
So the legs need to be max 113m apart along and photos a max 84m apart, along each leg, to ensure 1/3 overlaps.