I just tested this with greater expansion. Flawless Advance's tooltip displays the range as 16.9m. Standing ~25m from Hideout Abrams, I can hit him with Flawless Advance. After the lunge, Apollo is just under 20m from Abrams. Both of these values are significantly larger than the range displayed by the tooltip. My best guess is that it's measuring from the tip of Apollo's sword after the lunge, but as the ability hits from Apollo's starting position, it still hits a larger range than what's displayed.