Dear Sneha, in this link to Dr. Jun Jiao's lecture notes, you can find very interesting information for indexing SAD patterns:

http://www.web.pdx.edu/~jiaoj/teach.php

You can also find very good advice in this link to another question uploaded here in ResearchGate:

https://www.researchgate.net/post/How_can_I_analyze_the_TEM_diffraction_pattern

Please refer to my detailed answer. It will be helpful.

Refer to the equation :

R*d= L*Lambda = Camera constant (where R = distance from the center of the SAD pattern to the ring for which you are trying to evaluate d, d = interplanar spacing). When you carry out Transmission Electron Microscopy study please ask the operator about the value of camera constant. by calculating R from the SAD image you can find out d by camera constant/R. Please refer to my word file with a SAD image for more better understanding.

If you are interested in a software measuring the d-spacings from a ring diffraction patterns, you may find helpful CrysTBox (see the link below). It should automatically detect the rings, measure the d-spacings and crystallographically index the rings if the sample material is known.

http://www.fzu.cz/CrysTBox