Ok I will throw a value for pi in as well but before I do just explain why 22/7 and 34.56/11 (3.141818181).

The height of the pyramid 280 royal cubits x 22/7 = 880 twice the base however if you intend drawing a circle kor inscribing a circle into the base of the pyramid 440 x 22/7 = 1382.857143 not an even number. Using 3.141818181 makes it an even number. 440 x 3.1418181818 = 1382.4 or 691.2 x 2 = 138.4.

There are 69.12 miles in one degree and 24,883.2 miles in the circumference of the earth giving a diameter using 3.14181818 = 7920 miles.

Now Stephen your number 39.375 begin with the old Egyptian value for pi 3.160493827 and x 280 the height of the pyramid David you can begin with the number 440 and x 3.1416.

3.16049387 x 280 = 884.9382716 royal cubits we began with 280 royal cubits. There are 25 digits in a Royal cubit 884.9382716 x 25 = 22,123.45679 and there are 16 digits in a foot / 16 = 1,382.716049 indus valley feet 1520.987654 imperial feet base of pyramid then / 2 = 760.4938272 imperial feet 9125.925926 inches. (note socket length petrie 9125.9 same number?).

The perimeter of the pyramid is 30 seconds of 1 degree and therefore a degree =

760.4938272 x 4 = 3041.975309 perimeter 30 seconds

x 2 = 6083.950617 60 seconds or 1 minute

x 60 = 365,037.037 60 minutes or 1 degree

x 360 = 131,413,3333 feet 24,888.8888 miles circumference of the earth

/ Egyptian value for pi 3.160493827 = 7875 miles radius = 3937.5 your number.

David same sort of thing 440 x 3.1416 = 1382.304 / 2 = 691.152.

69.1151 miles x 360 degrees = 24,881.472 / 3.1416 = 7920 miles diameter of earth.

Jim