In general, you don't really zero the sensor - the sensor reads what it reads. What you do is sample the sensor and decide "OK, that's my zero", and save that value as the zero offset.
Thinking in terms of altitude, what you do is read the sensor, determine the absolute altitude (MSL, "mean sea level"), and assume that this value is ground level. Afterward, you subtract this MSL value from every altitude you measure to give you AGL (above ground level) altitude, which is what we use in rocketry.
When doing the launch detect code, remember it's not just a change in observed altitude, but a change in observed altitude over a given (short) time interval. If the weather is changing and you're on the pad for a while, you can actually see the pressure altitude change a bit while it's sitting there.
-j