IMO no more than 1/8 cup milk for 1# grated soap. I think 1# might be a bit too small for a rebatch, instead recommending 2# soap gratings and 1/4 cup milk, cow if ordinary soap or GM if GM soap.
I'm a newbie, blah, blah, just my beginner advice , don't add too much liquid. In any rebatch get by with the least liquid you can get by with. Adding liquid is only to prevent burning and to aid in some degree the soap melting and entering a liquid phase. If you can get there with the minimum of liquid you're all the better closer to a good rebatch without unnecessary liquid.
Speaking for myself I've had better rebatch luck with crock pots than with double boilers, but I haven't tried oven or boil-in-bag let.
BTW 1/8 cup milk is 1 oz, 2 tbsp., just as Laurie said. No more than that, ppo.