You're putting too much into this... I was approximating the reserve amount... If you fillup as you said you have been when your LCD is showing .9 you'll be fine...
Though if I wanted to fan the fire I could tell you that neither the fuel guage or the speedo are accurate... I've driven over 50 miles after it went to 00 on the guage... and now that you have your scan guage have the MPH on the guage display... the speedo is slow by about 5mph... the SG is accurate.