The opening angle of the Spectroradiometer is about 22°.
This avoids measurement of false lights, environment etc.
So please take into account, that you should move away from all borders of the cabinet ca. 20% of the height of the light source which you measure.
sin (22°/2)=0.19 =ca. 0.2 = 20% of height of lightsource should be the minimum distance from the edges . e.g.: if the light source is 1m above the SR the distance from the wall should be >0.2m
This distance shall also be used in front of the light booth to avoid to measure the ambient light!
Also most light sources do not fulfill light specs at the ends! (they get dark there and aged much more on the ends)