There are 2 ways to do this.
1, (The proper way) Measure a distance to the left of the 1st dart leg that is equal to half the distance between the 2 dart legs and place a node. Using the Flip by Line tool, select that new node, press enter, and then select the bottom node of the dart leg and then the top node of the dart leg. This will place a node at the centre between the 2 dart legs. Connect this new node to the 2 bottom dart leg nodes with a line.
When drawing your pattern outline select the 3 bottom nodes - you can draw the actual dart in as an internal path.
Using this method, you can also copy the curve at the waistline, if there is one, in which case, you can repeat the process on the other side and place a node where the 2 curve cross each other in the centre.
- (The easy, slap-dash way) The other way is to divide the distance between the 2 legs and then, using the Point at Distance Along Line tool, from the dart point node to the new node between the 2 legs at the bottom, place a node that is the same length from the dart point as the 2 dart legs are. This should produce a slight V, once you have drawn in the lines.
Once again, I normally draw in the dart as an internal path.