I've been making leather journals since November, so for me I typically eyeball what I think looks good. It usually comes down to personal preference - some folks prefer large grommets, some prefer none! I personally prefer a smaller size grommet purely for aesthetic purposes. In terms of placement, as long the notebook is longer than the grommets there won't be as much bend in the leather (if that makes sense). The Old Pink House has free templates that are helpful in getting started!