Back in the day when I did photography I remember reading up on taking photographs for legal use.  If I remember some of the concepts mentioned; Write down all the parameters on how the picture was taken, settings on camera, exposure, height of camera, how film (files) were handled.  Have reference item in the photos, if closeup's you would include a ruler if measurements helped. 
Have a plan on the photos, document and be confident in what you did with your photography.
I never did any legal photography but did some commercial work and always documented what I was doing mainly for reference for redoing jobs.