I have a paper on how to adjust your manual steering gear by feel in-car. It requires that you procede carefully since you must drive the vehicle after each adjustment.

Too much preload can result in your car having very little (or no) returnability (i.e. the steering wheel won't return to center).

The paper can be found at www.corvettefaq.com/A-car.asp (http://www.corvettefaq.com/A-car.asp) and is the very last one.

Here is a direct link: http://www.corvettefaq.com/acar/ManualGearAdjustInVehicle.doc

Thanks Jim, as usual you provide more valuable information!