OK. Fine tuned it.
(1)Create the layer. Delete in the property manager all the positioning stuff, left, width etc,
(2) Create a CSS style. Go to positioning
(3) Set your Placement/width at say, 90%
(4) Set your placement/left at HALF of the remaining %. In this case, half of 10%, or 5%.
(5) Apply it to your layer. It works for vertical postioning too.
Even easier? In the property manager set the layer width to 90%, and the layer left positioning at 5%.
Don't even need a style sheet.