How much extra decking to order to allow for offcuts
Ordinarily, I'd get 10% extra for a job like you describe. Given the custom setup, I'd seriously consider 15%, and maybe maybe if it wasn't outrageously cost-prohibitive, going for 20%. Splits, knots, and if you live in my world, mis-cuts happen. It would break my heart to have to go back for one more stick... 

Plus, if you get way too much, then you'll have the fun of figuring out a project on which to use the scrap. Bench seating? Table? Quadcopter? (Okay, maybe not the last...)