You are not working with concrete so don't worry about it. No slump test needed or squeezing some water out. The ingredients you use, the amount of humidity in the air all effect the amount of water so there no magic measurement. The consistency should be like thick oatmeal or thick cookie dough. After you've made 1 or 2 you'll see the difference. Too wet and it's sliding down the sides. Too dry and it won't hold together.

My basement, where I'm working, is VERY dry. Could that have something to do with it? Could it be drying too quickly? Do I need to be draping moist towels over it or sealing it in a bag for humidity?

Wait until the spring. Do outside when temps are min 50F.
Mix will cure best
Most important - avoid all the dust when in enclosed area. breathing the dust from the Portland is not good - unless you have a good mask.

Add water slowly - a little at a time. Mix it well. Take your time. it should look like hamburger or oatmeal - Then Take the mix and make a ball - squeeze hard - a few drips of water should come out. .

One other suggestion- Work the mix inside the mold, press the hypertufa firmly down and against the mold. It will take some of the air out of the mix and hold together better.

The amount of water changes with every batch; sometimes the peat moss is dryer, sometimes the concrete sucks up more water, maybe the perlite is drier some days than others; sometimes the planets are out of alignment lol. I have found it's easier to work with using hot water.

Like plantman said, take a handful of mixture and squeeze it; a little water should come out. Too dry, and it won't compress, too much water and it slides down the sides of the mold. Too much water also makes it weaker. Wetter mixture is easier to work with, but the down side is you lose longevity of your trough.

Put your just-completed trough in a plastic bag and seal it up to keep the moisture in. You can either leave it or unmold it in a couple days and reseal or put into a tub or water; your choice, but concrete cures in the presence of moisture, so just draping a towel over the trough may be letting it "dry out" and not "cure".