well, been at this full time since 1986, have always priced most walls, windows, boards, panels, box trucks by SQ FT. ive said this before, when you do sq ft pricing, you just dont use the sq ft of what you cover with vinyl, you USE THE SQ FT OF THE WHOLE SPACE.
you got a 4 x 10. the amount of vinyl/lettering will not be on it all. if your lettering covers 30 sq ft of the 40 available. BUT when you want to get paid like you should use the whole 40 sq ft to figure pricing. example:
4 X 10=40 sq ft @ $10 s.f. = $400.00 amount of lettering will be under 30 s.f. so if you charge for only what you do in lettering, this doesnt take in the weeded vinyl you tossed in the trash can. sq ft is the only way to figure most jobs.
boats, pickup doors, signs smaller 4 x 8, now these you need to figure sq ft, and then add for TIME, COMPLEXITY &PITA FACTOR)))