Well, the Massive Dev Chart suggests 7:30 at EI 50 or 8:00 at EI 100 for Perceptol stock; 10-11 minutes 1+1, EI 100; 14-15 minutes 1+3, EI 100. That should provide some starting points---it seems sort of optimistic to target box speed, but you could vary EI through a roll and see what works with those dev times, for a start.
Thanks! It's 4x5 film, already exposed at EI 50. I shall try 10 minutes at 1+1.