As a guide I would divide the time run by the distance ran to get time per metre.
Then multiply the time per metre by the distance of the race to be run.

Eg. If the two dogs in question were now meeting over 520M
Dog A (30.6) would equate to expected time of 30.3 whilst dog B (23.6) would equate to expected time of 29.2

Only a guide though as there are still lots of ifs and buts.
eg. Dog stepping up in distance may not be able to maintain metre rating (registered over the shorter distance)
Dog A may have registered 30.6 but received a number of checks in running, whilst dog B's time may have been achieved trouble free etc