You should have covered the whole tank! Lot better to have the whole surface covered in a mild solution, than the half of it covered in a stronger one! And, always agitate the solution very often. Use a windscreen washer pump, with the outlet hose placed into the fuel tap opening. Leaving the tank in its' normal position, let the tank fill from the underside, and overflow through the filler opening. This methode, flushes all the rust out from the inside. BTW; Did You check the inside of the top surface of the tank, afterwards? Use an endoscope camera for Your mobile phone. You'll be amazed how much rust is still at the top, when You think You're done!