Try to contact Sandy King who is quiet knowledgeable about the carbon process, there is also a carbon printing group on yahoo I believe. Rodinal works fine with HP5, since carbon is a contact print process you won't see much grain and with a 5x7 neg grain will not be visible on bigger enlargements. The neg should be quiet contrasty to quote sandy king it should have density range of 1.6 (print well on a grade 0 or 1 paper). So I would add some time say 25% to 50% to the published development times. But as I've said contact Sandy King he is one of the kings of the carbon process.

Bonne chance