This was debated some time ago, but who likes to resurrect old threads :/
If worker efficiency is your goal, then sure level 9 seems good - probably the same logic Nexon designers used. But if maximizing supplies gained is your goal, then you should consider that if each expedition has an equal chance to come up then on average for each expedition slot:
Level 9 yields 31+54=85 supplies in 4+8+16+16+33+4+8+33+33+16=171 hours or 85/171=0.50 supplies per hour
Level 8 is 0.26 supplies per hour
Level 7 is 0.65 supplies per hour <<< BETTER
That’s considering that you use each expedition. However if you only take expeditions for supplies (letting all others expire after 24 hours) then on average for each expedition slot:
Level 9 yields 31+54=85 supplies in 4+24+24+24+24+4+24+24+24+24=200 hours or 85/200=0.43 supplies per hour
Level 8 is 0.28 supplies per hour
Level 7 is 0.81 supplies per hour <<< STILL BETTER
If you really, really want supplies badly and don’t care about citizens efficiency then you can also take all expeditions less than 24 hours even when they don’t have supplies to expire them sooner. Then on average for each expedition slot:
Level 9 yields 31+54=85 supplies in 4+8+16+16+24+4+8+24+24+16=144 hours or 85/144=0.6 supplies per hour
Level 8 is 0.35 supplies per hour
Level 7 is 1.03 supplies per hour <<< BEST
(somebody should check my math)