I wonder know how to get the width of my string in pixels
If you use skins in UI it is a hassle to find out the correct font to feed into the GlyphLayout.
In this case I use a throw away instance of Label to figure everything out for me then ask the Label for the width.
Skin skin = getMySkin();
Label cellExample = new Label("888.88888", skin);
cellExample.layout();
float cellWidth = cellExample.getWidth();
Table table = new Table(skin);
table.defaults().width(cellWidth);
// fill table width cells ...
This is not the answer if you want to position the text yourself, but it is useful to make a UI layout stable and less dependent on the actual content of the cells.