1 Surface area of soffit:   sq. ft. 2 To determine number of squares of soffit needed, divide by 100.   sq. ft. Example: 267 sq. ft. Divide by 100 = 2.67 or 3 squares (Remember to use perforated soffit where ventilation is needed.) 3 Measure total linear feet of each accessory needed. Starter Strip linear ft. F-Trim linear ft. J-Channel (siding) linear ft. J-Channel (soffit) linear ft. Undersill/Finish Trim linear ft. Inside Corner Post linear ft. Ouside Corner linear ft. Fascia linear ft. Soffit linear ft. Shutters window height