Good Afternoon, Malcolm,
Going directly from the original concentrate is the most practical method. I use 1:7 generally; T-Max 400 gets 9 minutes normally, but, if the shots are done with flash on camera, I'll usually cut back to about 8 minutes. T-Max 100 gets 10 minutes normally, with a similar reduction for high-contrast flash shots. Both times assume 68 degrees Fahrenheit (20C). Also, both times are for 35mm and 120. When I use the same concentration for sheet film with continuous agitation in a drum, I usually reduce the times by 10 to 15 percent. Haven't used any HP5, so I can't comment from experience, but it should be fairly easy to establish a "normal" time with a couple of test rolls.