Use Tight when the day has little reliable room for deliberate work. One or two commitments can still make it complete.
Use Balanced when several commitments can coexist with the rest of the day. It is the middle setting, not the “correct” one.
Use Open when there is substantial room for focused action. Open means available, not unlimited.
Steady's suggested counts—two, four, and six—are boundaries for the primary planning flow. They help make overcommitment visible. They are not scientific measurements of human capacity, and they do not account for every kind of work.
Some days will surprise you. That is expected. Closing the day gives you evidence for the next capacity choice without turning one unusual day into a rule.