How to find the middle of a element and use the left float : somebody can help me?

I dont know how i can find the middle of a div and use the left float to set our Arrow in the middle of the div.

Screen:

[enter image description here][1]

---------------------------------------------

At the moment i "hard scripted" it with: 

    	$("#a_account").click(function(){//Account
		if(ucphideshow == 1){
			$("#userAdministration_div").show();
			$("#userCharacter_div").hide();
			$("#ucp_arrow").css("left", "46px");
		}
	});
	$("#a_char").click(function(){//character
		if(ucphideshow == 1){
			$("#userAdministration_div").hide();
			$("#userCharacter_div").show();
			$("#ucp_arrow").css("left", "125px");
		}
	});

Its still working but i want to use the middle of the div.
  [1]: https://i.stack.imgur.com/nzvgb.png