Exactly the opposite. Lengthening the strut rod would decrease positive caster. Positive caster leans the spindle back, so pushing the bottom of the spindle back by lengthening the strut rod would make the caster worse, not better. Shortening the strut rod improves positive caster.
Regardless, the strut rod length shouldn’t be used to tune the alignment specs. The most important function for the strut rod is controlling the movement of the LCA, so, if you have adjustable strut rods the length should be set so that the LCA moves freely through it’s whole range of travel without any binding.
