Put on a decent bra and the muslin and use a Red Pin to mark the spot about an inch and a half away from your bust point (the nipple). The dart should stop short of the nipple.
Okay, now that all the perverts are here, let's get on with it.
Here is the muslin laid flat. See the Red Pin?
Now I'm going to draw a line parallel to the existing dart. A see-thru quilting ruler comes in really handy.
Now I'm going to the inside and redraw the original dart in its new location.
Step 1, mark the same line on the inside:
Step 2, measure the width of the original dart and draw the newly located dart:
Or maybe, don't admire. I still see problems. Do you?