I wouldnt describe twice the meter fare as being "fair" though it may be small change to some ....

One technique I sometimes use in cities where meters are either non-existent or known to be dodgy is to write the negotiated price on a little post-it note and give it to the driver - a reverse receipt if you will. on arrival at the destination, there is no quibbling and any attempts to extort additional monies are easier to rebuff.

Another trick I used once was to re-route the taxi driver when I knew the meter was running too fast. I told him I had changed my mind and needed to go to [large hotel of choice]. Once there, I disembarked, told the bellboys what was happening and they got the fare reduced significantly.