The pixels used in margins are the same pixels used in absolute positioning. If you've used margins to move something then how did you work out the margin needed? You didn't use a tool for that did you? It's exactly the same process for absolute positioning.
If you want to move an element somewhere then you must know where you want to put it? If the div is 150px square and you want something of 50px square placed centrally then it doesn't take much thought to work out what the positioning needs to be (top:50px and left 50px). Indeed if the element you are placing is a fixed width and height you don't need to do any calculations as margin:auto will centre an absolute element vertically and horizontally when top,left,right and bottom are set to zero.
Here are both examples (which we have already been through several times already).
/* second example */
Don't make this sound harder than it is! It is simple but better than that it is easy.