I founded a VC fund and successfully backed over two dozen startup, seed and early stage ventures. There is a "new" type of security called a SAFE which allows you to raise money and to put off the valuation until the professionals come in on an A round. Failing that you can offer a simple proposition to the seed investors, a multiple and a round percentage. I have found that a 2X multiple and 1 percent per $25,000 is almost always compelling to both the issuer and the angel. Let me know if you need anything else.