Interesting factoid about numbers ending in 'illion' like quintillion - the prefix is the indicator of the number of sets of 3 zeros after 1,000:
1,000,000 - Million (though I don't know how "M" gets us one lol)
1,000,000,000 - Billion (two)
1,000,000,000,000 - Trillion (three)
1,000,000,000,000,000 - Quadrillion (four)
1,000,000,000,000,000,000 - Quintillion (five)
etc.
To your question, rather than throwing this sort of world building fact out at the beginning, maybe expressing it as a percentage might be easier:
"By the Fifth Millenium of charted history, researchers had discovered their predictions for planetary objects was off by 20%, throwing the number nearly into the septillions. Gallileo would have turned over in his grave. But this didn't prevent our hero, Spaceman Spiff, from hurtling through the vast nothingness in front of him for nearly 4 parsecs...toward the wrong planet!"