I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow? In general turbine engines are limited mostly by the

I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow? In general turbine engines are limited mostly by the temperature at the first stage of turbine blades after the combustion stage. It's hard to get reliable instrumentation in this zone so a proxy temperature is often taken from a later stage. In older airliners there was a table of max ITT values varying with altitude and outside air temperature. In modern FADEC engines the computer does the same calculations. It's not unusual for smaller jets to be set at their max continuous thrust setting shortly after takeoff and be left there until the beginning of the descent. Modern airliners will be flying a cost optimised Mach number. That Mach number is entered on their flight plan and is used as a basis for ensuring they are separated from other flights on the north Atlantic tracks so they wouldn't be able to change it readily. The interesting part of all of this is that they would have been assigned an altitude for the crossing, at high altitudes maximum speeds drop (because flutter margins are proportional to velocity rather than effective pressure) but stall speeds increase so the range of viable flying speeds is actually quite limited at cruise altitudes. They probably couldn't have slowed down if they wanted too.