Yeah, you can't get an accurate SG once you add the water. At least, not without an unreasonable amount of shaking. You can measure the concentrated wort and then account for the dilution, or you can just calculate the SG based on the potential extract (~43 point-gal/lb for DME). If your mass and volume measurements are reasonably precise, you can calculate the SG better than a cheap hydrometer will measure it.