I can tell you from experience that "perfectly" is not at all necessary. I'm constantly amazed at how well it compensates for my
sloppiness. How well aligned it needs to be at the start depends mainly on how far you need to travel (shorter the better)
and how close obstacles are on either side (closer the better [to act as guides], up to a point). But, overall, there's just no substitute
for experience here. Rest assured, the failure mode is "car stops too soon", not "car rams into wall".
Did you mean you can see how far from that wall you want the car to stop?
There's a summon/auto-park configuration screen that lets you set clearance distances. In my experience these are a little
counter intuitive: if you don't strictly need to allow a tighter tolerance, don't ask for it; larger tolerances provide better guidance
as your car goes in (YMMV, of course).