I generally balance out the arrangement with same colors distributed on all sides.
See from above the rounded state of the insertion of flowers. The flower heads are balances in both color and mass size.
Look for "openings" where you need to insert new flowers.
I have another orange carnation that I do NOT want to tower above the yellow spider - but it slides in at the height.
I still size the length of the stem by holding it against the existing flowers.
Looking down from the top, you can see where I plan on placing the next flower.
Here's what it looks like from the top - notice the yellow spider mum is still in the center and is the dominant flower.