My multiplier 'sweet spot' is 2000%, given where I'm at in terms of space on my map and item count. I've got around 2000%'s worth in storage.
I don't send characters on tasks outside of events, so rent collection is the main path to level-up donuts. I also have the Lard Lad Donut Factory and CostMo which combined return 12 donuts every 24hrs.
Using Sky Finger once a day outside of events nets around 24 donuts a day. I found buying Blood Mobiles and Rattrap Trucks to level up a hassle. Whenever I need more donuts I level up by buying and selling The World's Largest Zirconia, which is worth around 18 donuts a pop.
