It will take about 2 cups or 1 pound in that gallon Scuba. Use regular white table sugar heated in some of the juice to dissove thoroughly and then cool down before adding back to the rest. Check the SG and it should be close to 1.085.
Here is a chart from Jack Keller's site that might help you figure it out.
<CENTER>
<TABLE cellSpacing=5 cellPadding=5 width="80%" border=1 ="#ffff99"><T>
<T>
<TR>
<TD align=middle width="16.66%">Specific
Gravity</TD>
<TD align=middle width="16.66%">Sugar
in Gal.</TD>
<TD align=middle width="16.66%">Sugar
to Gal.</TD>
<TD align=middle width="16.66%">Sugar
Brix</TD>
<TD align=middle width="16.67%">Volume w/
Sugar Added</TD>
<TD align=middle width="16.66%">Potential
Alcohol</TD></TR>
<TR>
<TD align=middle width="16.66%">1.010
1.015
1.020
1.025
1.030
1.035
1.040
1.045
1.050
1.055
1.060
1.065
1.070
1.075
1.080
1.085
1.090
1.095
1.100
1.105
1.110
1.115
1.120
1.125
1.130
1.135</TD>
<TD align=middle width="16.66%">0 lb. 1.7 oz.
0 lb. 3.4 oz.
0 lb. 5.8 oz.
0 lb. 7.5 oz.
0 lb. 10.0 oz.
0 lb. 12.5 oz.
0 lb. 14.0 oz.
0 lb. 15.8 oz.
1 lb. 1.5 oz.
1 lb. 3.0 oz.
1 lb. 5.0 oz.
1 lb. 6.5 oz.
1 lb. 8.0 oz.
1 lb. 9.8 oz.
1 lb. 11.5 oz.
1 lb. 14.0 oz.
1 lb. 15.6 oz.
2 lb. 1.3 oz.
2 lb. 3.0 oz.
2 lb. 4.6 oz.
2 lb. 6.3 oz.
2 lb. 8.0 oz.
2 lb. 9.6 oz.
2 lb. 11.3 oz.
2 lb. 12.9 oz.
2 lb. 14.6 oz.</TD>
<TD align=middle width="16.66%">0 lb. 2.1 oz.
0 lb.4.2 oz.
0 lb.6.7 oz.
0 lb.8.3 oz.
0 lb.10.8 oz.
0 lb.13.3 oz.
0 lb.15.0 oz.
1 lb. 0.7 oz.
1 lb. 3.3 oz.
1 lb. 4.8 oz.
1 lb. 6.5 oz.
1 lb. 9.0 oz.
1 lb. 11.5 oz.
1 lb. 14.0 oz.
1 lb. 15.6 oz.
2 lb. 2.2 oz.
2 lb. 4.6 oz.
2 lb. 7.2 oz.
2 lb. 9.6 oz.
2 lb. 12.1 oz.
2 lb. 14.8 oz.
3 lb. 1.1 oz.
3 lb. 3.6 oz.
3 lb. 6.1 oz.
3 lb. 8.6 oz.
3 lb. 11.1 oz.</TD>
<TD align=middle width="16.66%">3.8
4.9
6.0
7.1
8.2
9.3
10.4
11.5
12.6
13.7
14.8
15.9
17.0
18.1
19.2
20.3
21.4
22.5
23.6
24.7
25.8
26.9
28.0
29.1
30.2
31.3</TD>
<TD align=middle width="16.67%">1 gal 0.7 oz.
1 gal 2.4 oz.
1 gal 4.0 oz.
1 gal 5.6 oz.
1 gal 6.4 oz.
1 gal 8.0 oz.
1 gal 8.8 oz.
1 gal 10.4 oz.
1 gal 11.2 oz.
1 gal 12.8 oz.
1 gal 13.6 oz.
1 gal 15.2 oz.
1 gal 16.0 oz.
1 gal 17.6 oz.
1 gal 18.4 oz.
1 gal 20.0 oz.
1 gal 21.6 oz.
1 gal 22.4 oz.
1 gal 24.0 oz.
1 gal 25.6 oz.
1 gal 26.4 oz.
1 gal 28.0 oz.
1 gal 29.6 oz.
1 gal 30.4 oz.
1 gal 32.0 oz.
1 gal 33.6 oz.</TD>
<TD align=middle width="16.66%">1.4
2.0
2.7
3.4
4.1
4.8
5.4
6.1
6.8
7.5
8.2
8.8
9.5
10.2
10.9
11.5
12.2
12.9
13.6
14.3
14.9
15.6
16.3
17.0
17.7
18.3</TD></TR></T></T></TABLE></CENTER>
Here is a link to where it came from. He explains about the sugars and use of the hydrometer.
http://winemaking.jackkeller.net/hydrom.asp
Edited by: appleman