Due to my natural sloppiness, I don't worry about turning final at any particular altitude; I just keep looking out of the cockpit and visually judging when/where I need to turn.
I often tend to come in slightly high on final, but a slip always takes care of that. If low (rarely happens), I add a touch of power.
In any case, it's best not to get in the habit of relying too much upon the altimetre for circuit decisions ... won't do you much good if you have to make a forced approach some day. The only instrument that I check once having turned base is the ASI (and VSI, if flying a glider).