So there's 8 tiles.
Radius from archway to the outer edge would be : 7 +1 = 8ft
You can use this formula to count the area of semi circle : π r^2/2
A = 3.14 x 8ft^2 = 100.48 ft ^2
A = 3.14 x 7ft^2= 76.92 ft^2
100.48 - 76.92 = 23.55 ft^2
Then you have to divide it with the total tiles
23.55/8 = 2.94 ft^2 per tile >> (rounded to nearest tenth)
hope this helps