1. It's basically information from a fuel summation unit, that integrates the fuel flow over time. Essentially what you see there is how much the fuel that has passed the HMU. You could have a leak before that, that wouldn't show on the FUEL USED, so good old airmanship is still required.
2. The N1 percentage on N1 LIMIT / TAKE OFF 1 page is displayed with regards to the actual pressure in the manifold, pack switches and engine bleed air switches positions.