Conventional hydrometers are skewed to measure salinity in sea water. Sea water is denser than the African rift lakes that you speak of. It may not be an accurate measure for the fact if you use other companies "rift lake salt" products, these products may be heavily influenced by sodium chloride. This heavy influence of sodium chloride will not replicate the lakes environment.
Our Cichlid Lake Salt has a relatively low chloride content, unlike other products, which is more conducive to replicating the natural environment of the rift lakes. Instead of using a hydrometer to measure total dissolved salts, I would be more concerned with GH (general hardness which is more specifically measured as calcium and magnesium), especially if using our Cichlid Lake Salt. GH test kits are inexpensive and can be found at most aquarium stores.