You can get all needed info from using the following method:
1) Take a sample of the must like 250ml or two cups or whatever
2) take an SG reading at this moment.
3) boil the sample until half of the volume has evaporated
4) Now add plain water until the exact original volume has been
reached again (250 ml or 2 cups or whatever, but exact the same)
5) Let cool down and take another SG reading
The SG reading in point 5 will give you the residual sugar (as all alcohol has evaporated)
6) Fill both starting SG from point 2 and ending SG from point 5
in fermcalc at the alcohol tab (boiling off method).
This will give you actual alcohol content !!!!
Now from the residual sugar and the actual alcohol content you can derive the starting SG.
If you would be using the metric system calculations would be easier
Please use the metric system
Now if you know the acidity also you would know everything there is to know about the original must.......