Let's get back to gridding with the flower stems alone rather than using the tape. It's faster and works well.
Place your second stem in the vase, leaning the stem in the opposite direction from the first stem.
Make sure that the stems are braced against the bottom of the vase as shown below.
Some florists wire the rose stems, inserting the wire into the caylex (bulb of the flower under the petals) and wind the wire down the long stem. This can help support the flower heads if the head is heavy and the stem is slender. This is totally an optional choice. Our shops did wire the roses and it's a habit for me to continue to do so.
This is the top view of the first two rose placements into the vase.