The goal to hemming a garment is that it is parallel to the floor.
It's not enough to cut straight across a garment to a get a straight hemline.
When fabric goes over the bustline it takes up more fabric so if you are large busted, you may end up with a high-in-the-front hemline because fabric is being used to go over your generous bosom!
The best way to do this is to have a friend help you pin up the hem and use the floor as your guideline and a very long ruler.
Once you have figured out how much longer your center front is you can use this is re-drafting your pattern.
Using the floor as your guideline for all your hems is the way to ensure a straight hemline!