To be able to make the farthest wall-jump (because you will usually need to do a long wall-jump to jump over a long barrel combination), the best location (on every platform) are to stand to the edge of the last small platform of every platform.
As you can see in Donkey Kong, the platforms are consisted of very small horizontal line, put on each other, to make the platform go up. So, the last horizontal line of every platform, you need to put yourself pretty much 1 pixel to the edge of it (the edge of it is the opposite direction of the invisible wall) before starting the wall-jump, in order to be able to do the highest possible wall-jump without landing on a barrel while touching the wall (because if you do a "too far wall jump", you will fall too low while bouncing off the invisible wall, and you might end up landing on a barrel while doing that.
So, by standing on the edge of this first (last) horizontal line of every platform, you can't go wrong with that!
(it was hard to explain, so hopefully you understand! haha. I could've put a screenshot to help explaining, so if you need a screenshot to explain, just tell it! ;-)).