The range and degree of flexion depends on the motion and gait of the horse.  You will see different ranges of motion while standing, and at the walk, trot, canter and gallop.  Conditions such as osteoarthritis may also limit range of motion (range of flexion).  I'd like to refer you to some scientific articles that you may find helpful:

1) Fetlock joint kinematics differ with age in thoroughbred racehorses, 2002, by  Butcher and Ashley-Ross.  Online link at: http://users.wfu.edu/rossma/Fetlock.pdf
These researchers measured fetlock flexion of galloping race horses aged 2-5 years.  There are tables summarizing data that might help. In galloping Thoroughbreds, it appears the angles of flexion ranged from approximately 97 - 106 degrees. 

2) Measurement techniques for gait analysis, by Clayton & Schamhardt.  This is a book chapter, online link at: http://www.elsevierhealth.co.uk/media/us/samplechapters/<personal data hidden>32/<personal data hidden>32.pdf
There is a graph on fetlock flexion on (page 59) of the trotting horse that you should find very helpful.  Flexion of trotting horses were reported between 20 degrees and extension about 50 degrees. 

3) Thesis of Ms. Jennifer Sanders, Auburn University, 2009, entitled, "The effects of two bandage protocols on equine fetlock kinematics."  Online link at: http://etd.auburn.edu/etd/bitstream/handle/10415/1643/EFFECT_OF_TWO_BANDAGE_PROTOCOLS_ON_EQUINE_FETL...

4) The University of California at Davis, School of Veterinary Medicine has some videos showing equine gain analysis online at: http://www.vetmed.ucdavis.edu/vorl/research_programs/locomotion_gait_analysis/equine_gait.cfm

I also suggest you check out the Michigan State University College of Veterinary Medicine McPhail Equine Performance Center webpage.  There is a list of publications that you may find useful.  The link is: http://cvm.msu.edu/research/research-centers/mcphail-equine-performance-center

I hope this helps.