Option 1:
Try on the t-shirt to gauge how deep you want the neck opening to be. While wearing the t-shirt, look in the mirror and mark the spot you’d like the point of the V with a pin.
Option 2:
Dig up your favourite V-necked t-shirt and use it as a guide.
Fold your crewneck t-shirt vertically in half, making sure that the back neck seam, side seams and shoulders line up. Do the same with your V-neck t-shirt and place it directly over the crewneck t-shirt. Line everything up as much as possible.
Make a mark on the crewneck t-shirt that corresponds to the point of the V on your V-neck t-shirt. Put your v-neck t-shirt away.