Good to hear from you again. Sorry I have been away so long.
Lelgemann cites a 20/28 value between the remen and the Nippur cubit, and he seems to think that the Nippur cubit came first and then the remen, and then the royal cubit, as remen x sq rt 2, although it seems just as likely, if not more likely to me, that the Nippur cubit came from the remen as 28/20.
As I understand it from the cited comments from Petrie and from de Lubicz in my article, the underlying digit value corresponds to the remen and not the royal cubit. The remen is actually 20 digits, or five palms, while the royal cubit is a little bit longer than seven palms, or 28 digits, to get to remen x sq rt 2, and this is reflected in some of the surviving cubit rods and also reflected in the digit scale drawing squares that have survived and that Petrie measured and commented on.
I think the correspondence of the remen with the length of minutes and degrees of meridian circumference is significant whether or not the remen came before the cubit, but the digit length make it look like the remen came first.
I agree with you about Userkaf, which is also a baselength of 198 remen, with half diagonal of 140 remen and diagonal of 280 remen.