Hebbal flat area calculations: What's the real deal?

Visited Mantri Developers' project near Hebbal last week. Kids have moved out, so we're selling our big house and want something manageable. But yaar, super area aur carpet area ka difference dekhke dimaag kharab ho gaya. Builder bol rahe hain itna, but actual usable space toh bahot kam lag rahi hai. Is this normal practice in Bengaluru? How much ka difference is typical, 25-30%? Aur ispe negotiation possible hai kya? Koi spot discount milta hai for ready-to-move flats? Want to avoid any hidden surprises later. Kya lagta hai?