Well the pitman arm should be pointing straight back when the gearbox is dead center. That puts center line 90 degrees perpendicular to the font axel’s center line, or directly parallel to the frame.
If your arm isn’t straight back when steering wheel is centered that means one of two things:
1. Your overall toe measurement is good but
In reality you driver side toe is actually positive like +3.5 degrees and your passenger side toes is actually -3.6 degrees giving an overall toe in relationship to each other as -0.1. But the functional toe to gearbox center is still +3.5 band -3.6. This means to drive straight your gearbox is in a partial turn and that means your steering wheel looks centered but is in actuality at least one turn from center
This is why I hate the overall toe method like with levels. And a measuring tape. It checks toe in relationship to each other. This might improve driving but is no where near optimal. The two string method compares to the average of all 4. But still to each other. The 3 string method actually sets it by the frame. If the frame isn’t warped will be true with the centered pitman.
2.You have an issue between the gear box and the steering wheel. Usually the gear box is off by one revolution on the input shaft which puts the pitman are 8 to 12 degrees off the pointing straight back with steering wheel centered. This happens all the time when Changing gearboxes everything wasn’t at center when doing. It’s why the service manual says to straighten wheels and secure the steering wheel when changing the gearbox.
You are NOT paying attention.