Your plumeria is unnamed. It could be the Celadine but until you see some mature leaves, you won't know that for sure. The Celadine has specific borders that are not seen on any other plumeria that I am aware of. Of course, the picture may have absolutely nothing to do with your plant. It may simply be a "stock" picture that the grower puts on all his cuttings.
I think this rooting-in-water is just so interesting. You (and the grower who produced those instructions) are going against everything I was taught and everything I have done for the last 25 years. I would say just cover the cut-end, perhaps with 1/2" of water, but so far, what you have done (6-8" of water) seems to be working. Don't feel bad. I have a couple of dozen cuttings that have been in perlite for 8 months and there is not a single root to be seen.
