TIL Starbucks coffee has a ton of caffeine. This explains why I'm super jittery every time I get Starbucks even though I drink (non-Starbucks) coffee every day.
So yeah, two freaking massive 20oz pours of highly caffeinated coffee will get you to around 1 gram.
But FWIW the daily recommended dose is 400mg. 1g is a ton of caffeine and way beyond what you would get to by sipping on coffee throughout the day (it's 10+ cups of drip).
I cannot find where the number originates, but there is at least one study @ https://www.sciencedirect.com/science/article/pii/S027869151... which explicitly claims "vidence supported consumption of ≤400 mg/day in adults is not associated with overt, adverse effects."
1g of caffeine is high, but with tolerance it doesn't seem _that_ high. I currently use 600mg/day to help with OCD[1], and it doesn't really effect me - heart rate barely moves, and I don't feel noticeably different. I've tried 1g, and it made me feel _slightly_ jittery, but that was about it.
[1] Counter-intuitive I know, but it seems to help - it makes the intrusive thoughts easier to ignore