The "spot" is constantly changing....unless that animal's standing still.
Wherever my pin is at the given time is the spot. I can't determine said spot until I get ready to pull the trigger.
Exactly. You cannot just say I am going to aim (
insert discription here) here. Your angleand the deer's position will change all of that. If every shot was broadside on level ground then yes a measurement would work. That's not how it happens though.