The # of Strings corresponds to the physical number of strings for that model and is generally 1. The lights per String represents the physical number of light nodes, bulbs or pixels.
The starting pixel is at the bottom of the circle. If physically your wiring is different, say it is connected in at the top, then you can rotate the image of the circle (by grabbing onto the top blue circle) by 180 degrees.
The Layers attribute value describe the concentric rings in the circle. This will create node count settings for each layer. The sum of the node counts must add up to the number of nodes in the model.
It is also expected that each ring will be wired in a circle, with the outside ring first and then the next inner ring wired and so on.
The Center % is used to indicate how much area the empty area of the circle occupies.
Decreasing it will drop the circles inwards and increasing it will push the circles outwards.