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.
Defining a valuation for a seed round for a startup can be pretty difficult. It depends if it's pre-product, pre-revenue, what market etc. It will also depend of the size of the round itself.
Would love to help you if you let me know more.
SAFE is definitely the best way to go if you're pre-revenue. Post-revenue, you can use exit multiples on top-line revenue to get close. I'd recommend having the investor suggest it and use a market-valuation if you don't want to set it yourself.
It's difficult really unless you have record of valuations of other companies who are in the same industry when they raised a similar round or had similar milestones completed.
Keep in mind a valuation isn't really a precise science but it's a measure of what the market is willing to pay to buy the equity you are selling.
As pointed out, there is no good answer based on business logic. What I would do is to ask the lead investor if there is one, what target multiple the want to obtain on their money, a 10 X is low, and when they expect to obtain it an how. The you can ask if the 1% per $25,000 will do that for them.
Avoid any attempt to do it based on projections, even if you have some revenue. The product will probably change so as to invalidate such a "promise".