I do the eyeballing method- basically, I spritz my grated soap with enough water to just make them moist (i.e., not swimming) then I start heating in a 180F oven (covered). As it's heating, I peek every now again to see how it's doing, and spritz more water if things look too dry. Then I repeat the cycle of peeking/cooking/spritzing until it reaches the right consistency to glop into the mold. Doing it this way prevents me from being too heavy-handed with the water up front. Adding a little SL to the spritzing water doesn't hurt, either.