Bear in mind that the smaller you make this length, the higher the forces on your linkage, and hence the stronger you'll have to build it. The smallest value I've seen commercially is on the S-327, at about 40mm, and they've used some fairly mighty aluminium track rods. This is how you make the steering arms point inwards, the classic way to achieve Ackermann geometry. Not surprisingly, this defines the position of the pivot for the handlebar assembly. I've assumed you want to keep it on the centreline of the trike.
Negative values cross the track rods over.
This is the distance from the main handlebar pivot to each of the pivots for the track rods. Increase it to get more steering movement from less handlebar motion, or vice versa. C29 Handlebar offset This is the distance from the midpoint of the handlebat to the main handlebar pivot. If you increase too much, you'll find the ends of the handlebar swing in and hit seats etc much faster than if you'd kept it small. You should now have a first-attempt setup.