How do you figure the number of 4 x 4 vinyl tiles that you will need in a room.

I'm in the process of installing the self-adhesive tiles in my bathroom. Everywhere I look for tips on how to start the project recommends finding the center of the walls and snapping a chalk line to make 4 quadrants. This looks easy in all in the pictures because there is nothing else in the floor, but in my case one wall has a built-in counter extending half way and the other wall is occupied by the tub and more cabinets. I guess I don't understand how to start this if the area to be tiled is not a square or rectangular area. I would appreciate all suggestion and comments. Thanks in advance Pete