Thats a tough question to answer. I know I spent my donuts as I got them/bought them over the years. I never really had any "reserve" till I hit around 2k%
Im currently sitting at 3000.01% with no intention of going any higher. Ive got around 800% in storage that I just dont see the need to put out. Ive been trying to buy higher % items (ex whale @ 7%) to replace multiple smaller % items (ex flags @ 0.25%ea, so 1 whale = 28 flags) I'm doing this to reduce my item count before it becomes a problem.