In twelfths I just measure the life-size thing, work out what it will be at 1/12th and crack on replicating it the best I can. Here's the resulting exercise for a rose from life to 1/48th.
|photo so I know how they look in a vase|
|photo of a bud|
|photo of a bloom|
|photo of a whole stem to measure|
|click on photo to enlarge|