If 1.2 gallons covers 225 sq ft then 1 gallon will cover less area of course.
It will cover an area equivalent to 225 times the proportion that 1 is to 1.2.
1 divided by 1.2 = .883
225 times .883 = 187.5 sq ft
That example was used just to illustrate the proplem
I would just do it this way:
If 1.2 gallons covers 225 sq ft then 1 gallon will cover 225 divided by1.2 = 187.5 sq ft