It looks very simple to use the following relation:

d=L*Lambda/r

d diffraction distane of hkl plane

where L is the camera length (should be calibrated using reference material)

Lambda is the electron wavelength at a working electron accelrating voltage (can ce calculated or experimental which is recommanded)

r:direct measurement on the SAED between one diffraction dot and the central beam or to minimize the errors between 2 dots in that case you get 2r

alos check the bibliography of Oberlin Agnes who published several paper on

Also the book on Transmission Electron Microscopy by Carter tome 2 is a reference book on SAED techniques and more

Calculations are reliable as soon as your TEM is perfectly configured and all the working parameters checked and certified by operators