How many acres would you spray if you sprayed the perimeter of an 80 acre field with 90 foot booms?

That depends on the shape of the 80-acre field.

However, if you assume that the 80 acre field is the standard one-half of a quarter section (a common US field shape), and that there are no roads, right-of-ways, or correction lines to contend with, then you would spray 15.62 acres by going around the outside of the field once.