Ease is not always easy, unfortunately. Calculating how much ease to add to a garment trips people up. Like, if your model has a 40″ bust, what should the bust measure on the actual tee shirt, blouse, or coat?
My answer is that it depends on your design and the fabric you are making it in. Some designs are made to fit close to the body and use stretchy fabrics (think activewear and swimwear) that use negative ease. Some designs are meant to layer over others (like a coat) and use bulkier fabrics that need more ease to be comfortable and fit the design intent.
Ease is not universal either. Different areas of the body require different amounts of ease to get a comfortable fit or to fit the design silhouette.
The best way to estimate what ease your design needs (if you don’t have a good pattern block to start from) is to find an existing garment that you like the fit of that uses similar fabric to your design and measure it against your fit model’s body measurements.