I don't think that's right. Underground infrastructure provides a set colony cost of 3.0, not 4.0, so 300 UI provides space for 1 million people.
Also, UI is only valid on bodies whose gravity is too low to colonize in the first place (displayed as "N/A" on the "Colony Cost" column in the "System Generation and Display" window). Any uncolonized body that has a numerical colony cost is ineligible for UI. Try SMing in some UI on Venus, for instance. The game treats it as regular infrastructure, negating the fixed colony cost of UI.