I know a lot of gas flyers and I don't think any of them adhere to the 12" rule. Most of them strategically place the throttle servo for best operation. My throttle servos are usually in the area just between the fuel tank and engine or right to the side of the front of the fuel tank.

I make a servo box or plate and mount them with less pushrod bend as possible. I don't like slack in my throttle setups and some of the longer rods don't work as well. The smaller planes I just place them were they fit best.