by my calculations for a 90 x 90 using 4in cut squares (3.5 finished) , you would need about 662 squares. that's 90 / 3.5 = 25.71 and 25.71 x 25.71 = 661.11
80 x 80 would be 80 / 3.5 = 22.85 and 22.85 x 22.85 = 522.12 squaares.
i think that is about right. good luck and have fun.