every 40 rolls you do in gnw or time limited treasure grant you 1 fragment in average (is actually a chance to be 1 or 2, with 50-50 chances, every 60 rolls).

lets consider the 800 rebate vs doing in 3 months twice the 300 and once the 200 rebate.

the first 300 grant you the same outcome, so the comparison is between the 5 frags you get by hitting 800 and the 66+36 additional seal scrolls the rebates the following two months will grant you.

102 seal scrolls are, in average, 2.5 frags.

so, by doing the 800 rebate you actually get 2.5 more fragments in average.