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.