To know a plant, grow a plant!
Why did you decide to add this additional metric? Has it supported your height data?
We decided to take in cotyledon width along with height, because at the time we were unsure of which trait we were going to be selecting for. Basically, we wanted to have more options for when the time came to select. Then the data taken in that we didn't use for selection would also be later used to find any correlation to the selected trait.
A photo of our plants with five days of growth can be viewed at this link.