An alternate approximation would be to take the original volume and divide it by the final volume to get a reduction factor. Then multiply the reduction factor by your initial points value ((SG-1)*1000). So,
RF = 5 gal/5.125 gal = .976
74 * .976 = 76
Convert back to SG is 1.076
I know it's different than what others have said, but just wanted to throw out a different way to do it that has always worked for me.