If I understand you correctly, you want to parent a pin to a layer.
Obviousvly the best workflow would be to add a null and copy/paste the pin position to that null. (delete all keyframes on null position). So they are on the same spot. Now you add this expression to the pin:
L = pickwhip null here
And finally you parent the null to the layer you want.
The null will follow the picked layer, but with it's offset and the pin will follow the null's anchorpoint mapped in worldspace (which is the same space a pin lifes in). Since null and pin are on the same spot, there is no jump or offset.
Another workflow is:
Add your pin and the layer to follow. Add a keyframe to the layers position (another keyframe on the pin is already added).
Now you read out the position values on those keyframes. Since they are keyframed, those values won't change, aka this is the static offset you typed in all the time.
Finally you subtract the offset from the current position.
myPos = pickwhip pin there;
myPosStatic = myPos.key(1);
targetPos = pickwhip position of target layer here;
Layers with Puppet Pin applied should not be scaled, moved, or rotated and you cannot animate any of those properties. The position of a puppet pin is based on the layer dimensions, not the comp space. A simple layer space transformation is not going to work. If no parenting is applied you just need to correct for layer positon. If you wanted a pin to follow null 1 the expression would look like this:
If the null is parented to something then you'll have to add in a layer space transformation, but this will fix position errors if the puppet pin layer has been moved. If the layer has been scaled then you will need to add in a factor for scale.
The best workflow is to just make sure that the layer with puppet pin applied does not have any position, scale, or rotation properties are at their default values. Just reset everything before you apply Puppet Pin and you won't have any problems.