We are going by the calendar date... but one of our customers Ashley of hither and thither blog is marking her boy's growth by monthand I think that would totally work too!
Another customer suggested that you can paint each 12" in different colors to customize the stick so that you will have an idea about a range of actual height as little one grows. There isn't really a right or wrong way of leaving the mark on LIFE's Journey... and I hope that the product is so simple that each family can customize THEIR Measuring Stick to work for them.
We are planning to install another LIFE's Journey when our baby girl arrives. One measuring stick on each side of a door frame:)