There is no magic percent. Have your Realtor do a Comparative Market Analysis (CMA) using Sold comps within a 1 mile radius of the property (the closer, the better) that have Sold within the last 3 months. This will give you current market value and that is what you should base your offer on.
The reasonable percent of listed price is whatever it takes to adjust the price so it's competitive with other houses. And in line with what's recently sold. It could be 0%; it could be 25%. Point is: You need to know the comps. Then you make your offer based on that, not based on some percent--reasonable or not--of the listing price.
So what if it's $30 per square feet more expensive than other homes. You say "comparable." But does that take into consideration all the factors going into a comp? Not just bedrooms and bathrooms. Not just overall square footage, or size of the lot. Not just the age or the condition of the property. Not just whether the property's near a highway, underneath a flight path, or something else. Unless you're a builder, forget price per square foot. (Some here will disagree with me on that, but there are way too many factors affecting price that aren't reflected in price per square foot.)
You've overcomplicating things. Forget some set price reduction. Forget price per square foot. Determine what the house is worth, based on comps. Then pay no more than that. Probably offer less. Your Realtor can help you with that strategy.
Hope that helps.