When you tuck the shirt in, at the very front the two sides of the shirt tails separate and make for a gap in the shirt right below your belly button. The lack of alignment can make the shirt look sloppy.
If the last button on the shirt is above the belt, you need an extra button at the bottom of the shirt. Increasing the shirt length to the next threshold point will cause the shirt to be made with an additional button at the bottom of the shirt tail. With Proper Cloth, the number of buttons on the front of a shirt is a function of the shirt length as well as the top button placement. When viewing one of your saved sizes, as you change the shirt length or the top button placement you will see the calculated number of buttons on the shirt front.
Number of buttons on shirt front
The length of the shirt will affect the number of buttons depends on the front according to the following logic:
Shirt length = 26 to 27 inch shirt length has 6 buttons
Shirt length = 27.25 to 30 inch shirt length has 7 buttons
Shirt length = 30.25 to 33.5 inch shirt length has 8 buttons
Shirt length = 33.75 to 37 inch shirt length has 9 buttons
Shirt length = 37.25 to 39 inch shirt length has 10 buttons