thinking about the direction =thinking about the distance
because any change in distance will make change in direction
or you mean that when make the cable direction parallel with humerus , is the only element in determine if the position right or no???
This is all unnecessary in my opinion. Using free weights will solve this problem. But, just stand near the cable machine, then the resistance will be pulling mostly downwards. If you go further, it might change the resistance of the bottom poisition in stuff like bicep curls. But THIS IS COMMON SENSE. You wouldn't go 5m away from the cable machine. That would feel stupid and be stupid. Wether you stand 45cm from the machine or 1m from the machine it has very little significance.