Hi,
A very good and age-old question. The others have provided good benchmarks that I've seen recently. I'm working with a few start-ups right now that are Pre-Series A.

From a number crunching perspective, you should build a financial model with discounted cashflows to arrive at a valuation. A good model will let you fiddle with your key strategic assumptions to give you a range. Potential investors will run the numbers so you're better off having your own as their starting point.

Have you considered using a Simple Agreement for Future Equity (SAFE)? It's an instrument born out of Y Combinator for early stage companies. Like it says, it simplifies the process and doesn't force you to go through valuation negotiations and gyrations just yet even if there is an implied valuation.

Another thing to consider is limiting the investment that any single investor can put in. I am assuming that the last thing you want right now is to yield control to others.

I am happy to talk you this if you set-up a call. I can help you with all of the above and more.