Re: "I have never carried a footprint but I'm doing a 20 day hike this summer and was just thinking of 20 straight days in a tent on granite might do some damage so trying to figure out the best way to bring the most lightweight option."
Then, as others have said, a sheet of polycro is rather light as well as dirt cheap. It's a perennial favorite here. And it would work fine for granite, which I suppose is more of an abrasion threat than a puncture threat. Stepping on your floor on granite is, I think, what you're worried about?
Re: "Guess I've not drunk my full cup of the ultralight kool aid."
Well, there's kool aid and then there's KoolAid. I'm quite willing to point out the unfortunately-named "stupid light" when I see it but this one is not, I think, so much an ultralight issue as just a backpacking issue. Footprints are scams, full stop. Well, for 99% of users. It's a way to wring a few more dollars out of some victim who just paid $450 for a tent and is now anxious not to destroy their investment. If the floor is so remarkably more fragile than the rest of the tent that it needs a footprint, well, it shouldn't be. That's not a good design. I didn't use footprints back when I was non-UL, either. Heck, I still have my old Half Dome- somewhere- but no footprint for it. Of course, nowadays I just use a floorless mid (usually with a bivy) and thus avoid the issue.
I might buy the idea in especially odd situations like camping on a scree field or something. Or (why?) someplace with a lot of glass debris. In which case I'd take a cheap piece of polycro. Otherwise… no.
So, drink up!