DD-X is a speed increasing developer and most use it as so. This may be the reason why Ilford does not recommend DD-X for pulling HP5 below box speed. I know many have gotten beautiful results pulling HP5 in high contrast conditions using Perceptol. FP4 and HP5 in 120 are my two main films and I develop them in ID-11 1:1. FP4 shot at 100 when spot metering, or 64 using in camera metering. And HP5 shot at 250 in camera with high contrast and 640 in camera with low contrast then increased development time. You need to run tests to determine your optimum development time for each film and developer. I wouldn't get too caught up in determining development times for each EI. There should be one EI and development time for FP4 in normal contrast and then another for N+1, N-1.. etc if you wish. Same for HP5. If shooting with in camera metering your EI's will be lower than if you shoot with spot meter using the zone system. This is the reason why many times different photographers have extremely different EI's for a single film. Some using spot metering, some use in camera metering.