Applying a standard percentage reduction to any offer is not the most strategic way to make an offer. You might get lucky, but if you really want the property, I wouldn't rely on luck.
Study the comps carefully to formulate a fair offer. Then determine your best and highest offer based on the comps vs. the actual property, and make an offer below that so there is room for negotiation.
If you want to attempt to stay as below as possible from your best and highest price, be sure to structure your offer so that it is as attractive as possible: few or easily removed contingencies, nice down payment, preapproval letter, solid earnest money, and quick close.