There were a few sets of common numbers, formulas, and mental calculation tricks that were useful to just always have in the back of your head. Perfect squares and cubes under 1000, powers of 2 and 3, prime numbers below 100, interior angles of regular polygons up to 10 sides, binomial coefficients, Pythagorean triples, factorials up to 9!, common roots out to 3-4 decimal places, and a few others that I've no longer needed for almost 20 years at this point.

    Now I, even I, would celebrate
    In rhymes unapt, the great 
    Immortal Syracusan, rivalled (sic) nevermore
    Who, in his wondrous lore,
    Passed on before,
    Left men his guidance
    How to circles mensurate.

That is (after consulting a table), 1 decillion - 1.

I clearly come from from a short scale culture. If I switched to the long scale and Chuquet names I could, of course, go to 999 nonilliard, 999 nonnillion, 999 octilliard, etc.