I think conditioning for clamping is more important than conditioning for hanging. And I don’t think reducing the jelk time is necessary.
Clamping creates tremendous pressure on the arteries, veins, and tissue of the penis. Jelking gets your penis in shape for handling increasing pressure. After you get used to jelking, you can add squeezes, bends, uli’s to further condition your penis, and increase the pressure. I also like to clamp the base of my dick with one hand while I jelk with the other, to get good pressure. Once you build up gradually like that, then you will be physically ready to clamp.
When I clamp, the pressure is so high and my dick is so engorged and rock hard that jelking doesn’t even seem possible. But sometimes I will add a little bit of a bend, or a squeeze of the head while clamping to increase pressure.
When you are ready to clamp I think 20 minutes is too long. I would go with something like 3x10 minutes. Shorter sets, but more intense.
For hanging, just start light, keep the sets at 15 minutes or less, and watch for head pain and bruising.