I always keep an eye on my actual length of the inserted rose by placing it next to the vase and gauging how much I need to cut the rose down.
The inner roses are a little taller than the outer ring, because it's impossible for all these stems to rest on the bottom of the vase.
This is the naturally formed dome that I'm trying to visualize each time I cut a rose to size.
Rotate your vase around to be sure all your roses are evenly spaced.
Now all the roses are in place. My center rose still leans a little to the left, but I'll correct that when I insert the greenery between the flower heads.