Never measure the first flower. It is usually larger than normal.
Never measure last flowers. They are usually smaller.
Measure from sepal to petal if that is the largest area across.
Take measurements from more than one flower.
Take measurement of 'typical' flower shape. This is tricky. My WHAT ABOUT ME shows two shapes, the latter one being recurved. I like to show more than one bloom of most of the intros so that typical is seen at different times! Whatever that means!!!
