The sweetness of your sucralose and acesulfame potassium mixture is 0.45*600 + 0.55*200 = 380
Since allulose has 0.7 (or 70%) the sweetness of sugar, one kg of allulose is as sweet as 700 g of sugar.
So you are "missing" 300 g of sugar.
But you want to add the "missing" 300 g of sugar as your sucralose & acesulfame potassium mixture, which is 380x as sweet as sugar, so you only need to add 300/380 g which is around 0.789 grams.
I think that is more or less correct, but you might want to ask the question on Math SE anyway to be sure.
