So that rough double rmn is composed of 3.5 parts per remen ?
20 / 3.5 = 5.7142 fingers.
Do you not know a remen is 5 palms and a double remen would have 10 divisions ?
a double rmn is 40 fingers = 30 roman uncia (1/12th part pes) = 29.17" or 741mm (not 27.5" or 698.6mm)

Now children lets see what could be cut into 3.5 parts....
Small cubit = 6 palms
Shoulder (Egyptian rmn) = 5 palms
Dsr = 4 palms
Large span (pD aA) = 3 ½ palms
Small span (pD Sri) = 3 palms
[www.touregypt.net]


The cubit is 4/3 longer then standard.
It is cut into 7 parts
1 of those parts is cut into 1/14th.

Royal cubits are cut into 7 parts and 28 parts.

Now you can think that the rod is cut into some unknown fraction of a ridiculously large remen or use a bit of savvy...
5.333' finger lengths and 2.666' finger lengths
5.333' fingers = 1 hand = 4 inch
2.666' fingers = 1/2 hand = 2 inch.

A hand has always been 4/3rd longer then a palm.
An inch has always been 4/3rd longer then a finger.
A foot has always been 4 palms of 3 inch or 4 fingers
A foot has always been 3 hands or 4 inch.
A foot has always been 12 inch or 16 fingers
(except pes naturalis at 3/4th foot and celtic/ploughmans foot of 12 fingers et cetra)