So how exactly did you calculate the chance? As far as i know, only drop rate for the non-mem voucher made public was the one from Astral and Crimson Orbs which is 1%.
For all we know, it could very well be 1% across all quests that can drop it, in which case the most optimal quest to farm it with would be the one that's fastest to complete.
From my perspective, you've just picked the quest with best results for you and made it look like the absolute best here.
But anyway. From my results alone, i'd suggest "The Assistant" & "Supplies to Spin the Wheel of Chance" quests.