Calc Children of Time - The Old Empire's maximum interstellar flight speed

Children of Time isn't too heavy on feats, but it's one of the best science fiction books I've read in a good while, so I feel the need to share it and give it more of the attention it deserves. Let's start with a feat that's outright given to us...

Twenty long years for her signal to reach Earth. Far more than that for the rescue to come back, even with the best fusion engines employed to accelerate to three-quarter light speed. But her frail body would survive thatlong in cold sleep—and more than that.

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Old Empire ships can travel up to three-quarters of the speed of light, or 75% C.

Final Results
Maximum interstellar flight speed of Old Empire ships = 75% C

Also as a note, it's not actually the Old Empire; that's just what people who came later called it.