Beginners should always work through a full ROM, full stretch to full extension. This should, obviously, exclude locking at the elbows and knees, where you risk joint hyperextension. Straight, but not locked.
You can get away with tweaking your ROM to more of a functional ROM rather than a full one, since you've got the experience to use it to increase difficulty rather than a way to make things easier.
Also, for the flared elbow pressdown/cable cgbp, try doing it with a close-grip pulldown handle, with knuckles toward each other (hands facing apart and out). Start with elbows wide and forearms about parallel to the floor, then push out and stop about 2-3 inches short of full extension. Keep all the reps quick, almost piston-like. If you need to rest from rep to rep the weight is too heavy. 15-30 reps. Good shit.
