What resistor to choose for circuit?
The calculation is awkward, but correct.   A '6V' battery is 6.0V when new, and 4.0V when nearly discharged.  So, a resistor can achieve the 0.7A current only while the battery is fresh.   

Some non-resistor options exist.   AMC7140  is an integrated circuit, one of those will keep the current steady at 0.7A with no
resistor required.