As your product is on idea stage, the risk is very high and so the shares of the investor will be. While starting up, the best option is to self fund your business if you can, because in seed investment, most people end up giving a large percentage of share in return of a small amount of money, and that causes problem when the startup need series B funding. Typically, you probably wont be owner of your own company and That is a hard fact.
You can refer to this http://www.forbes.com/sites/alanhall/2012/09/09/you-want-how-much-ownership-in-my-business/
Is says if your business is on idea stage, investors can rightfully ask for 50-80% of ownership.
If you can bootstrap your business, that will be the best option, or if you really need investment, you can first wait for the offer and then propose yours.