Graham:

It puzzled Petrie as to how the builders were able to square the structures to such accuracy with an abundance of core material remaining in the center of the structures.
When setting a foundation you must confirm it square by diagonal measure or you may end up with a trapezoid or parallelogram-shaped base.
In effect, you are drawing two right-angle triangles and comparing their hypotenuse lengths (diagonals) until they are equal.
There is nothing complicated and that’s exactly what the AE did, but they didn’t have to measure from diagonal corner to corner as Petrie assumed. The alternate method is drawing an “external” right-angle triangle along the full length of the structure.
If the base is 440 units long (G2) then you “make” the hypotenuse (440+1) unit in length and then you “make” the short side equal to the sqrt of (440 +(440+1))…or a number when added to itself an equal number of times to produce the 881 value…the number is 29+17/15ths. You now have a 440-unit side; a hypotenuse of 441 and where the short side of 29+17/15th intersect with the hypotenuse is the formation of your right angle triangle.

Best.
Clive