I usually center the text in my labels and then widen/shrink the size of it to fit the highest possible number (i.e. 100%) the label will display. Reason is, if you left or right justify the text and have a shadow outline it will cut it off in game. Unless you're using a custom font, the default font you see in the game is wider than that you would see in UIBuilder. Centering vertically avoids having the font overlap anything above and below it (i.e. if you have a tight frame around the label) such as the ^^^ you would find in the target window for example. Another idea could be to remove the shadow outline completely if the text is going to be on a dark background constantly anyway.
