>can you give the dimensions of the postulated-intended chamber?<

According to who's measurements you use for the pit in the floor of the chamber it would have been around 25.5 feet high by 46 feet by 27.5 feet.

Using Petrie's suggestion that the floor level would have been at the point that the descending passage met the chamber if it was extended gives a figure very close to 25 feet on my original drawing. So both these heights are rather more than Petrie's "lofty" 20 feet.

Best of luck with the dig and thanks for the recomendations.

Jon