Try this one. Are you on Inventor 2012?
Its as simple as it gets now..
Creating as iFeature (I use as Punch even if not sheet metal) make it as easy to place and size as any Hole feature.
What I do is on my slot sketch I have a center point. When I constrain I turn on the slot sketch and constrain the hole to the center point in the sketch.
It's good practice to put a workaxis at the centre of any slot. Try this workflow that's detailed in the animated .gif attached for a tidy way of placing the axis.
edit - you need to save it then right click on it and open with Internet Explorer
An extruded surface body cylinder in effect creates an axis with less work and more functionality (Insert constraint now available).