Multiply width of the back of the shirt by two to find the size of the whole shirt. Now subtract your bust line measurement from the entire shirt width. The number you are left with is how much you will need to cut away from the shirt.
For Example: You bust line is 36" and the back of the shirt is 20". Multiply 20" by 2 to get 40". 40" - 36" = 4". In this example, your cut-away measurement would be 4".
Start your measuring tape at the line you drew down the middle of the shirt. Measure half of the cut-away measurement from the middle line. Mark with chalk. Measure from the other side of the line and mark with chalk too, like in the above photo. Make these marks all the way down the shirt. Use these marks as a guide to cut away this extra middle material.
Now you will sew up the back of the shirt.