Project Allocation rules

Hi,

Is there a way to set up Project Allocations to distribute Misc Transactions between projects without specifying an expenditure org or expenditure type? Our business model uses expenditure org for cost center and each cost allocated could be different. Based on what we found by trying to create an allocation rule, we would need to create a separate rule for each possible combination of cost center and expenditure type. I do not see any mention of overriding these values in the allocation rule. Am I missing something or are the rules that specific?

Thanks in advance for your help.