I am designing a small bathroom. I have a vanity then toilet then bathtub.

I know that toilets need to be 15 inches measured from the center line, but that confuses me.

Most toilet bowls are 28 inches wide. if you measure from the center line that leave once inch between toilet and the other two items.

What is the ideal space between a toilet bath tub and vanity with the toilet in the center? (Not based on the cente line but actual distance measuring from outside of toilet?

Additional Details

What is a center line ? That doesn't make sense. The toilet size has to matter. You cant mesaure 15 inches from a hole in the floor then put an 18 inch toilet over it and have a teeny space between the toilet and tub?