The fall x 3 is 55.5 and this translates your original diagram
777/55.5 = 14
1221/55.5 = 22 but for the sekhed take 1/2 of this for a ratio of 11 to 14.
You get 22/14 or 1/2 of modern pi.
Is 51.84 actually the angle found by petrie at al for the angle of slope?
I am familiar with the fall because i spent some time looking for the Holy Rood at some of the Scottish sites around Edinburgh. Edinburgh casttle to Holyrood house is 5920 feet this is 53.333r Holy Roods of 111 feet.
This is an eclipse calculation system based on 53.333r.
The holy rood is the rood x 6 so 18.5 x 6
The English equivalent is 6 x 16.5 = 99
5280 / 99 = 53.333r