Lizanator-  The location is very specific on a horse but you have the concept correct. You either measure from the outside corner of the eye (lateral canthus) to the base of the ear OR inside corner (medial canthus) of the eye to the poll.  I recommend placing tape so that you know you are dead on.  A .22 is recommended if you have one.  To know they are dead is very important.  The eye will become fixed and dilated.  This means they will stare straight ahead and the pupil will be wide open so all you see is black.  If you tap their eyeball they will not retract the eye or blink.  The heart will continue to beat for a little while so that is not a reliable indicator.  Be prepared for the horse to start to thrash anywhere from 20 secs to 1 minute after the shot.  This is completely normal and is the release of electrolytes from the dying muscle cells.  Most importantly, if any of the eye signs have not happened, shoot the horse again.  It is difficult but necessary.  If that is too overwhelming ( I would not be able to do it with a gun, I would be looking for a hand-held captive bolt ), call someone like others have mentioned.  


A good reference site- http://www.vetmed.ucdavis.edu/vetext/inf-an/inf-an_emergeuth-horses.html

For anyone reading that prefers a gunshot/captive bolt vs euthasol, you can also buy a captive bolt (you do not need to be a vet, producers buy them for livestock) to keep on the farm.  http://www.hogslat.biz/Blitz_Captive_Bolt_Stunner.asp