You should determine the mounting point yourself. Sit in the car with the lap belt secured and measure where the buckle sits while the lap is as low and tight as you can make it. That is the purpose of the 5th point belt, to hold the lap belt down so it can't slip up over your hipbones. Your installation instructions will have a range of acceptable angles for the 5th point belt. Use that geometry to find where place the mounting point and the seat slot.
But please do not take this advice without consulting and diligently following the instructions from the manufacturer. If you don't have a copy, they are available online.
In theory, there is no difference between Theory and Practice. In practice, there is. - Y. Berra