There is always confusion about such conversions. To really understand them one has to look into the origins of the two measurements. RCH (red pubic hair) originates back to intercourse between the British Screw Association (BA) and British Standard Pipe (BP) that declared several such measurement units . RCH (red pubic hair) was used for all visible measurements and the ubiquitous SMH ( Short mickey hair) for much finer tolerances.
The fleas dick unit , as you would know , was conceived by Dick Whitington, the inventor of the Whitworth system as an attempt to provide a standard that could be easily checked anywhere in the world by the use of the standard flea.
Less well known , the inclination angle of the appendage, was was in fact the basis of the 55 degree thread angle still with us today on our beloved Whitworth fasteners , and copied on by all proper fixings such as Brass Threads, British Standard Conduit (BSCon), Model Engineers (ME), and British Standard Copper (BSCopper).
Both terms , unfortunately , have defied all attempts at direct conversion into metric, although there was a rumor that a conversion table had been prepared by the French Meccano company based on MHS ( Milli Hair Standard.)
It lost something in the translation and never took off.