Sfumato had it right. The answer depends on when you start sweating, which is why it's a meaningless question. If you're single, don't give a **** about your retirement and have no significant obligations, then live it up, 50% or more can go to a car. If you're married, have kids with college expectations and hope to travel and be comfortable in your later years, then the standards will be stricter. 2%? 5%? No more than that probably. You will reasonably get every answer in between or beyond and there's a reason why for every answer.
Diffeerent strokes for different folks. It's you're life, do what you can do and still enjoy life. I play it pretty conservative.
Sagacity and philanthropy are not the primary motivators when purchasing a Ferrari. Much more base emotions are involved. More gutteral. To quote the great philosopher Tim "the Tool Man" Taylor... " 12 cylinders ...arrr arrr arrr arrr "
I think I used 90%, don't take investment advice from me! I did get laid 4 hours after owning the though.