MJ:

There are samples of Royal cubits having the 15th digit divided into 16 parts.
If this were a standard practice of dividing their rulers then, as you claim, the smallest measure would be 1/448th of a Royal cubit...or22/1000 of our system...or 0.022Rc.

Using the Kc dimensions, you agree with Petrie's claim of 20.63 inches representing the true value of the cubit length.

Using the coffer height, nobody agrees with Petrie's 20.66 inches representing the true value of the cubit length.

Using the base measure of the same pyramid, I agree with Petrie's 20.61 inches representing the true value of the cubit length.

Using the eastern wall width of the Qc (side with Niche) produces 20.57 inches.
Using the base width of the Niche produces 20.58 inches.
Using the depth of the Niche produces 20.54 inches.

From max to min = (20.66-20.54) or a 0.12 inch discrepancy.

This indicates the builder's accuracy in measure to be 1/6th of a digit at best...right?

Unless there is a specific reason why the Kc and Qc have different values...think white/red.

Best.
Clive