To be fair to your question let me answer it this way.
If you bought all your flowers you needed for the week on Monday, you would have to project what your sales for the week might be. After your Monday's purchase, it doesn't really matter what any individual arrangement sells for. What's far more important is whether you generated enough revenue for that week to cover all your costs.
It's just a different way of looking at things, but if all goes well my way, I maximize the use of labor, while minimizing my overhead costs. all through high volume.