Hard to say based on numbers what you will get, but for what it's worth:

My Patriot birth cert. said 311fps @ 70#, 29"d/l and 350g arrow. I shot a 363G arrow @ 73# and 29" d/l and got 300fps on the nose. If the graphs look the same in terms of arrow wt versus fps, then I'd say your predicted numbers (255-260) will be accurate.

Chrono it and let me know.

Fritz