Speed is relative to a frame of reference, so my understanding is that percentages of impulse are about power, not about speed, the 0.25c limit is a safety limit against a fix reference (maybe the center of the galaxy?) to reduce the relativistic effects, and does not require power to maintain. Effectively, with “full impulse” you are asking for the maximum power output until your speed relative to the galaxy center is 0.25c. Given in this example it is said we need to keep at 1/8 impulse because of power constraints I can only assume this is referring to power, not speed. With no reference frame and time component, none of these dialog make sense whatsoever, but we should be used to writer not understanding physics by now 😬