A good offer is the amount you are comfortable investing in order to own that property. There is no set formula or rules that dictate how much to offer based on the asking price.
A short sale doesn't mean cheap. Even in this market, the property will not sell for an absurdly low price. However, short sales can mean a good deal for the buyer, and even the seller. Short sales though, are always carefully reviewed by the bank, and even a full price offer may not be accepted.
When making an offer, ask your realtor to run comps on the property, and base your offer on what you can afford, what similiar properties have sold for recently, and be ready for your offer acceptance.
One tip for your and your realtor to consider, a short sale offer isn't "better" because it is for more money. I have had many short sales sell to lowest bidders because of things like, short escrows, large deposits, or all cash offers. There are many factors to take into account when deciding what price to offer, ultimately it has to be based on your desire to own that home.