Hi Toby,
From your question I assume you know that Perceptol stock solution doesn't deliver full box speed. But it does seem to when diluted to 1+3. So at a dilution of 1+3 I would rate FP4 at 125 and HP5 at 400. However, I really like to be taking photographs in hazy sunshine, so my films get reduced development to keep negative highlight densities down. This also brings midtones down a bit. I like these nice and bright, so to avoid them getting suppressed I give an extra stop of exposure, i.e. rate the film at half its box speed. This brings the midtones back up to a nice level.
I have to say that I think FP4 in Perceptol 1+3 gives a wonderful tonality. There is no compression of midtones when you do the compensating development as described above. Instead you get nice bright and well separated midtones.
People talk a lot about grain and sharpness. But when you have a framed picture hanging on a wall, you have to go right up to it before you are aware of grain and sharpness. But the tonality is plain to see from right across the room.
Sorry to go on.....
Alan