This is a pattern based on a website I found once upon a time, that sadly no longer exists, so I am recreating the drawing and measurements that I use before I do something stupid and lose my paper and never be able to sew an apron dress again.
You need three measurements
1) Around the widest part of hte bust + seam allowance
2) Length from chest to waist (from top of where you want the apron dress to where the flare should start)
3) Waist to hem (from where flare starts to however long you want)