Your horizontal (or virtual) top tube length should be derived from your torso and arm length and riding posture, so I can't understand why a fit calculator should prescribe this using your leg length. And horizontal top tube length should be the same whether you're riding a traditional, compact, or semi-compact frame. It's a virtual dimension, meaning it corresponds to no single component of the frame. So if 56 is your horizontal top tube, select whatever nominal size matches this.

Generally, 56 cm horizontal top tubes are found on bikes whose nominal size is 54-56 cm. In most brands, that's considered medium to large. This would be an appropriate size for riders with average proportions who are taller than 5'8" but shorter than 6'.