If a measurement doesn’t say rounded, heaping, scant, or whatever, it’s always level.
Mark Bittman / NY Times says one tablespoon each cornstarch and water per cup of liquid. I’m not sure why to bother with water, I’d just use the braising liquid.
Just go little by little until you are happy with the consistency? And agree with @robert to skip the water and just mix the starch in a bit of your braising liquid.