Why diluted, not basic
Basic shares understate the future denominator for any firm that issues stock compensation. The 10-Q EPS exhibit gives you both; use the same denominator as the company uses for diluted EPS.
Buyback math
A firm with flat absolute FCF can still grow FCF per share by shrinking the denominator. A 3% net buyback per year compounds to a 16% reduction over five years.
SBC dilutes the denominator
SBC issuance increases shares outstanding; buybacks-to-offset-dilution close the loop but only if the buyback dollars match SBC dollars at the prevailing share price. See SBC in FCF.