would be used at level flight? (At an altitude of 12,000 metres)

Would this be the pretty much the correct altitude for going from Dublin Ireland to Barcelona?(the flying time is just under 2 hours)
flýing an airbus 300

Not enough info. what airplane

about 89.0 N1 would give you around mach 0.74 or so. Thats about normal at 32 to 38K altitude.

Depends on the limits for the engine. Either will be N1, N2, or temperature limited. Most likely N1. Depends on company policy for power settings/efficiency.

Aircraft engines r usually operated much closer to full throttle while cruising than u might operate the engine of a car while driving down the road, but this is by design. Automobiles have a power reserve because in some situations u might have to suddenly speed up, even on the highway (overtaking someone else, for example), but u never have to suddenly speed up while cruising in an airliner, so no huge reserve of power is needed. The engines have a reserve sufficient to keep the plane flying safely (but more slowly) if an engine fails, but other than that, cruising power is pretty close to full power, since u generally want to fly as fast as practical (that being the whole advantage of flying). In other words, airliners tend to fly close to their maximum speed. They r still capable of flying at much slower speeds, but the whole idea is to fly fast, so u get to ur destination quickly.

Cruising altitudes depend on direction of flight & weather conditions, but the most economical & practical altitudes r usually between 30,000 & 40,000 feet. Shorter flights often have lower cruising altitudes than longer flights, because it requires a lot of fuel to climb to high altitudes. Cruising at high altitudes uses less fuel, but if the flight is short, the fuel cost of climbing to a high altitude is greater than the savings u get from cruising at that altitude, so cruise altitude might be a bit lower.