I'll repeat; every shot into every pocket has a thick entry, middle and thin entry. Being able to alter one's aim in these minute adjustments will require quite an accurate aiming system and we have one.... it's called the brain. Let's say you've got a shot where you need to "throw" the ball into the cornet pocket with max English and stun for the maximum throw so you can hold the CB. Hitting thick on the pocket is an important ingredient in order to hold the CB is we're at the max.
I guess I started with some sort of ghostball. I'd place my tip behind the OB, on the shot line for the pocket. I'd place it on this line in a spot where I could see about an inch of cloth between my tip and the OB, assuming this would put my tip close to where the center of the CB should be. From there I would keep my tip planted and walk my cue over to the CB and see exactly where I should aim, not at this tip spot but directly on or near the object ball. I didn't know it then, but I suppose I was aiming at specific fractional aim points. Within a week or two of doing this I didn't need to hold cue out over the table anymore, and I didn't focus on any particular aim. I just saw the shots and shot them, adjustment here or there as needed, whatever amount felt right.
As far as playing right or left of center pocket for particular CB position, I only do this when the OB is within 2 or 3 feet of the pocket, and rarely do I need such manipulation. Settling for a slightly less-appealing leave is better than trying to play perfect shape, only to barely miss a ball by trying to pinpoint it's path into the pocket. Close-up (couple of diamonds or so) is no problem, but farther out is asking for a miss.