If you decide to use Perceptol at stock I'd say that EI 250 is very close to being the right speed. I am surprised that neither Ilford nor the MDC offer times for RPX for Perceptol at 250.

Have a look at other films such as Tri -X and HP5+ at 250 in Perceptol and check these times against APX 400 at box speed and see if a 10-15% reduction gets you close to the suggested Ilford and Kodak film times.

I'd be surprised if the two figures are not close. Perceptol tends to be a slow working developer with quite long development times so if you are in the 10 mins or more period then a minute either way isn't going to make a critical difference.