No one can accurately tell you how much you need to take without knowing what your "baseline" SG level was before you start diluting. The only way to know is to order some validity test strips so you can monitor your SG before and after diluting. The test strips are found dirt cheap by doing a Google search for drug adulteration test kits.

Aspirin is old tech. Back in the day, it was used as an interferant for only one type of test: the first-generation EMIT screen. Since then, a second-gen EMIT (EMIT II) was introduced to counter the interference issues caused by aspirin. As a result, the first-gen EMIT is seldom used anymore (if, at all).