Replace 0.701 with a number field that is set to auto-enter 0.701 in the data box. When the need arises to change the value, change the amount auto-entered. Existing records will continue to use the 0.701 value. New records will use the new value.
Since this Total calualtion is a stored one and is reffering to the fields of same table, then you can make the calculation like this
If (not IsEmpty (Total ; Total ; ((Coef+Base)*Net)/0.701 ))
What this calculation will do, when it finds there is some value in Total field for any existing record it doesn't replace the new calculation but for new records the new calculation will be evaluated.
Hope it will works for you.
Thnk You so much! :)