It is taken from Thom's 3rd book and is important in that it introduces Richard Heaths value for the megalithic yard, estimated by Thom at 2.722 imperial feet.
It is important to understand the implications of this within the GP design assuming 1111.111r variable megalithic yards in the GP baselength.
Jim wakefield has 2.75 feet as his meg yard and Thom 2.72
At the GP base they produce 36666.666r inches and 36266.666r inches so 0.03 inches in a megalithic yard produces 400 inches at the GO base and this equates to 4 solar days of 100 inches.
2.722 produces 3024.444r feet being 36293.333r inches - 36266.666r = 26.666r inches being 2.222r imperial feet
3055.555r - 3024.444r = 31.111r imperial feet.
This becomes using a unit of 1.111r feet exactly 28 units.
So Heath's megalithic foot is placed exactly 28 units from the GP maximum baselength.