If your hands are X inches apart and You left hand becomes Y inches higher than your right hand the horizon sits at an angle of Z degrees
The larger X is the smaller Z is for a given Y
Now consider a constantly changing Y and you will see that the closer the hands are together the more that wobbles of your hands are translated to angular movment of the lens, known as camera shake
You try and keep your hands at the same level but you are a wobbling heap of muscle and bones - and in my case fat - run by a brain that is looking after all sorts of proccesses mainly concerned with keeping you safe -
not a tripod
Talking of tripods you will be aware that smoother pans are used by using the pan bar which puts your hand distant from the lens, that is well established, same concept
If you go for really long handles - down by the waist -you get another issue - panning - you are pushing from under the camera and tend to rotate it as you push to pan
Pan right and the horizon drops on the left
Ideally your hands would be the same height as the lens (or more correctly the CofM of the camera) but that is tiring
it will depend on what rig you have and how you have it outfitted.
its going to be a matter of what feels best to you.
but in general, for comfort and stability, you probably will want to keep your hands as close to your body as you can (probably 8-12 inches out from your chest)
laterally, one grip about middle of your chest and the other about even with your right shoulder.
with the weight of the rig slightly back-heavy. so youre not lifting too much with your arms.