How to set a specific location of a Prefab within the child of a Canvas?

I'm looking for a way to Instantiate a Prefab at a specific location within a Panel. Here are the specific details:

• The Panel is using anchors at X(0, 0.8) and Y(0.4, 0.8), so the size of the Panel will change as the screen size changes.

• The Prefab is a Transform with one Child that contains a Transform and a Sprite Renderer.

• The Canvas 'Render Mode' is set to 'Screen Space - Overlay'.

• I'm trying to do this with a number of different Prefabs of various sizes.

• The Scale on everything in the Scene (as far as I can tell) is (1, 1, 1).

• I've been using: thingy = (Transform)Instantiate(PrefabThingy, new Vector3(0, 0, 0), Quaternion.identity); to Instantiate the Prefab, then setting the thingy.transform.position to a different Vector3, outlined below.

I've tried a couple of things without success. If I don't set the Prefab's parent to the Panel, it ends up in the middle of the screen with Transform position (0, 0, 0). If I set the Prefab's parent to the Panel, it ends up at (0, -385.6, 0). Setting the Y value to 0 puts the Prefab way off-screen.

When I move a larger Prefab to (1, 0, 0), it hardly moves. When I move a smaller Prefab to (1, 0, 0), it moves a great distance. This is happening even when the Prefabs both have the same Panel parent. Both Prefabs have Sprites with the same Pixels Per Unit.

I'm confused as to where (0, 0, 0) is relative to each of these GameObjects.
I'm also not sure why setting two objects to (1, 0, 0) seems to put them in different places, even within the same Parent. How is this related to the size of the Prefabs?

I would like to know how to set the Prefabs both at some relative distance from the edges of their parent Panel (say, 20% from the left edge) and how to set some absolute distance from the edges of their parent Panel (say 50 pixels from the top).

Any help would be greatly appreciated!