The other way to do this (and the one that was used by Ballard Camera, the lab I took my color to when I lived in Seattle, prior to 2003) was to increase temperature by the equivalent amount. With a correction factor of 4% per degree F, you'd raise your process temperature by 3 degrees F to be equivalent to adding 30 seconds to the usual 3:15.
The advantage of this, for their setup, was that the leader card machine had a fixed process time and this was the only way to increase development. At the end of their work day, they'd raise the temp by this amount, run all the day's Push +1, increase another 3 degrees and run the Push +2, and if there was any Push +3 they'd raise the temp another 3 degrees for that. This way, the machine had all night to restabilize the temperature for the beginning of the next day's normal process. Down side was, if you wanted a push, you'd get your negatives and prints back the following day instead in an hour or so -- but it also allowed them to process push without a surcharge, because it was almost no extra effort for them.
