http://www.avc.com/a_vc/2011/04/how-to-allocate-founder-and-...
Essentially, figure out what market rate is for their salary. Since you'll likely pay a discount rate, the equity should bring them at or above market.
So if Market is 105k, and I'm going to pay 55k, I will give you 50k in equity (based on a reasonable valuation--assuming you don't have an actual valuation).
Using a flat rate percent can get tricky, and you can easily end up unnecessarily diluting yourself.
I also think it's important to think of your first 1-3 hires as Key hires, the same way a new CEO or VP of Sales would be a key hire post Series-B. You want them to get a big chunk of money if there is a positive liquidation event.