oh, sorry, i thought you were using briess as your approximation.
hopville has "sorghum syrup" as an ingredient and lists is as having 37 PPG. so 90 oz = 7.5 lbs in 5 gals = 1.056 OG. assuming a 15% boil-off for one hour and you boil one hour, you'll need to start your boil with 5.75 gallons, which in turn works out to 1.048 due to the dilution of the extra 0.75 gallons. these number will change if: you're doing more than a 5 gal batch, your evaporation rate is different than 15%, you boil for more or less than an hour, and what the actual PPG of your sorghum syrup is.
for that last point, you could dilute some in water and use a hydrometer to figure out its SG, then calculate out what that means for the whole 90 oz.