My preference is between 4 and 5 o'clock and 7 and 8 o'clock for the left and right tanks, respectively, with 9 o'clock and 3 o'clock being where the knobs are located. I do this with steel tanks and aluminum tanks. As for the length of the leashes, it depends on your attachment point and your a$$ width. If you use a butt plate then a$$ width will be more of a determinant than anything else. Butt plates do vary in width depending on the rig, but you cannot get a narrow versus wide butt plate for the same rig. If you are attaching the tanks to d-rings placed on your waist strap then the length of the leash will depend on where you have the d-ring positioned. When using aluminum tanks you'll have to figure out where you want to position the front position d-rings and then position the rear position d-rings so the same leash length will work in either position.
So, in other words, there's no one answer. It depends on rig, attachment point, tanks, etc.