More interesting to me is the chance of intelligent life variable. A couple of months ago on Ars Technica iirc, there was published a writeup on a paper on how macro life (much less intelligent life) requires something analogous to mitochondria for the cell to be large enough to handle that level of complexity (the problem being caused by the geometric relationship between volume and surface area). the paper goes on to say that the chances of one organism entering another and them then entering a mutually beneficial symbiotic relationship as being astronomically low (it is, however, a function of time, but Ars implied (iirc) that this doesn't noticably help the Drake equation the chance is that low). Based on this I would adjust Fi by a couple dozen orders of magnitude
On a scale broader than single cells there are around 80% of vascular plants and their symbiosis with fungi, the bacteria in our guts, the worms close to black smokers, certain sea cucumbers that absorb algae and live off their photosynthesis, and much more.
