Duster REAR bench seat material yardage?!?

hello fellow enthusiats, i have a 70' plymouth duster and i'm going to get some custom made seat covers for it(primarily for the color preferance i have). what i need to know is how much yardage of vinyl should i purchase to have a rear seat cover sewn up for me at an interior shop? they sell vinyl but the yard for about $45.00 so i would like to not overspend but not buy too little. any advice would be fantastic i believe and rear seat in a dart, a valiant, a duster, or a demon would be similar. thank you all in advance!!!