OK, I managed to save 76% on Level 4.
(I don't get the exact buliding thing :question )
Levipak3 - Level 4: Gas distribution facility
80 Lems
75% to save
RR: 1
Time: 6 minutes
Skills: 1 blocker, 35 builders, 4 bashers, 1 miner, 6 diggers
Good: You have to optimize your route to save the left side.
Bad: It's actually rather straightforward (the easiest route is the most obvious imo) and a bit long.