Feeding alfalfa cubes dry is no more a risk for dehydrating your horse than your horse eating his regular hay dry.
Some horses do tend to choke on the cubes, some horses do fine.
Soaking is not required but most people do soak them.
The bonus with soaking is it is putting known water into your horse.
(It is not really extra water because studies have shown that most horses simply drink less if there food is made wet.)
I would look at the alfalfa cubes as part of your hay ration. How many pounds of hay is your horse getting now? Your horse should get 1-2% of their body weight per day in hay (obviously this can reduced if on good pasture and such).
If you have one of those plastic grain scoops, I believe those hold a total of 3 pounds of grain. So I would start off with only about half of that.
Or...See if you can find an old soup can or coffee can that has about 16 ounces in it, that is equal to a pound. Its a cheap way to measure too.
Fluid ounces does not necessarily transfer over into a weight. Fluid ounces is a volume measurement not a weight measurement.
And saying a scoop holds three pounds of grain is very over generalizing. Plastic grain scoops come in various sizes. I have one that claims to be 2 quarts and one that claims to be 3 quarts and they are just about the same size. If I fill it with pelleted feed it will weigh quite a bit more than if I fill it with an extruded feed (like most senior feeds).
That is why people really need to weigh their feeds to get an idea of how much they are actually feeding.
And no, I do not expect people to weigh every feeding. Sure that would be best but who has the time for that...
Once you know what a pound (or six pounds depending on your horse) looks like you can use your scoops like you would normally use them.
If you do not have a scale at home (digital kitchen scales are pretty cheap as is a fish scale that you can get in the sporting department at Walmart to weigh your hay) measure some hay cubes (or grain) into a clean baggie and take it to your local Walmart store and set it on the scale in the produce department.