I must say I think you may be overthinking this. Just use Lenght x Width = sqft,sqyds, or sqinches, which ever works for the size of your layout. If it is multiple images as you state than once they are cut you can simply measure the vinyl used to get your total amount. It is not worth wasting the time to try to save a little vinyl.
Once you have your sqft you can figure out how much to charge for the material. You have to remember there is a lot of waste in doing vinyl.
Good luck.