Working on RC prints the same size as the final FB prints can work very well as long as they are very similar emulsions such as Ilford Multigrade FB and RC. The emulsions will of course always vary, usually only slightly, but if lots of dodging and experimenting to do, will be quicker and cheaper on RC to get the feel of the print. You then have to work out what changes to be made when switching papers but it can be enough to give you a good head start once you start on the FB. Just finished printing 20 16x20s for a client using this method and in this instance I had to add 12% to all the different exposures. However, when I opened a new box of 16x20 with a different batch number, had to work out a new percentage!