I have a simple question about casting mechanics that might sound complicated, because I'm not sure the exact best way to ask it, but here it goes...

If there is a buff on me that gives me increased healing and I cast a heal spell... at what point is the heal with the buff calculated? As soon as I hit the button to begin casting the heal or after the heal is actually cast and lands?

This is for macro purposes. See I'm not sure I would gain the increased healing, if say, I cast Physick, but then my Eos casts and lands Fey Illumination mid-cast of Physick, then the Physick goes off. Would the Fey Illumination had to have been applied before I hit the button to start casting Physick? Thanks!