The technology of hydrometers make them inaccurate by nature.
-> Required plug for hydrometer. Can get them fro $40 new, or cheaper from awk-shun sites. <-
Your second-best bet is going to be taking three to five readings with each hydrometer, and taking the average of all the readings. This should keep you from overcorrecting any issues due to swings you'll see on the hydrometer.