There are lots of ways to measure a building. The highest occupied floor is 287.1m but Urban Toronto typically measures to the top of the mechanical/roof which is 298.1m. The tip of the antennae is 355m.
All of these are measured from lowest public entrance from outdoors. Due to slope of the ground, these points can vary by a bit depending on how you choose the "bottom" point.
IMO, buildings should be measured from the lowest occupied slab, often the lowest underground parking level, to the highest architectural element but nobody aside from me does that: roughly 310m for FCP. A building on an above ground parking podium shouldn't be encouraged.