suburban parking standards tend to be 4-5 per 1000sf

If you are looking for the specific requirements under zoning, contact the local planning department.

But Alan's estimate is typically what I've found, too.

yeah but i'm working in an urban setting. it isn't worth it to contact the planning dept. over a school project

besides i think the amount of parking per sf is an inverse proportion, and i'm working to near big-box standards

big box retailers are going to want 4-5 cars per 1000sf. they dont care if you are in an urban setting or not. that's what they want. and since they are typically the anchor tenants that make retail projects happen (ie: smaller retailers won't sign on without the bigger retailer draw in place), the anchor tenants tend to get what they want. you can make the smaller tenants do unconventional things, but its generally hard to make the anchor tenants operate outside of their formulas, 'cause if they decide its too much brain damage, they walk, and you got no project. this is way much of America looks they way it does - 4-5 cars per 1000sf is more square footage given to cars than to buildings.

that being said, if you are in an urban setting, with dense mixed use development, street parking, etc, you might be able to squeeze the parking req'mts to 3 per 1000sf for big box retailers. unless you are in Manhattan or some similar hyper dense environment, big box retail isnt going to operate too far beyond that. (a good mixed use compact pedestrian oriented district like downtown Santa Monica or Boulder will operate at 1.5-2 cars per 1000sf - but generally such places do not include big box retail.)

some good resources: http://www.nelsonnygaard.com/articles.htm and http://www.linemag.org/_line (latest issue is on parking)

i have been working on a master site plan for a "shopping center" and the county requirements are 1 space per 300 square feet for retail. 1 per 200 for restaurant.

If you want to be realistic about it then contact the zoning department. This will usually just take one phone call, so I don't see why it's "not worth it" for a school project. It should be much less time consuming than following a long discussion here...
Even if you find that you have to go to zoning in person that's not a bad idea. As a student I found that the zoning people were very friendly to students and wanted to tell you the whole history of zoning and planning in their city. Plus if you talk about having gone to zoning to find realistic parking requirements, future employers' ears will perk up. Employers are always complaining that students have zero interest in real-world practicality and zero experience with real-world codes, zoning, etc.