Will you point out where it says that? I really did find this paper unusually cryptic about concrete takeaways.
Table 2 suggests that to match the performance of studies 3 and 4 (done by other researchers), which each gave 600 mg of caffeine, they were able to cut the dosage down to 500 mg and 400 mg respectively. But it's not clear to me that they're recommending that as the overall optimum, and that's the only place I'm finding "600" in the paper.
Table 2 suggests that to match the performance of studies 3 and 4 (done by other researchers), which each gave 600 mg of caffeine, they were able to cut the dosage down to 500 mg and 400 mg respectively. But it's not clear to me that they're recommending that as the overall optimum, and that's the only place I'm finding "600" in the paper.