You said in your post you were paid competitive salary. Why is it 65% less now? What are you basing your salary on?
From what you've said, it sounds like the mathematical models play a pretty significant role. If the product is like Digg, then the algorithms are 90% of the company (in fact I think K. Rose outsourced the site). If the product is like Myspace, however, then your contribution will be the other way around.
In your situation, the best bet is to negotiate your equity compensation based on opportunity cost. I recommend getting the negotiation finalized before the company does find a hire, since this gives you more leverage. I say start with 3% and negotiate downward.