I'm going to be making a loaf of bread in a 900 g tin (11" x 4.5" x 4" deep).
What is the total weight of the dough I will need (when mixed) for a tin this size?
Literally speaking, a 900 g (2 lb) loaf tin is designed for a 900 g loaf. You'll be fine if you use that much dough.
Some mass does get lost during the baking process due to evaporation, so if you want to end up with 900 g and not less (as an actual bakery would try to do), then you want to plan for anywhere between 8% and 12% loss, depending on the kind of dough and how long you bake it for, as well as how quickly you cool it.
The standard figure for mass loss seems to be about 9%, plus whatever is lost during cooling - generally around 1-2%. I would compensate 10%, meaning to use 1000 g (1 kg or 2.2 lbs) of dough.